Editor placeholder in source file

newText is an Optional String, so you need to unwrap it, or provide a default value. Maybe...

textFieldStrings[index] = newText ?? "some default value"

Other than that, you are using several variables where you don't show the declarations.
So there could be a problem with one of those?

When you post code, don't post in comments. It is totally messed up.

Here it is formatted:

class ViewController: UIViewController, UITextFieldDelegate {
    @IBOutlet weak var benchPressPB: UITextField!
    @IBOutlet weak var squatPB: UITextField!
    @IBOutlet weak var deadliftPB: UITextField!
    @IBOutlet weak var ohpPB: UITextField!
    @IBOutlet weak var rackPullPB: UITextField!
    @IBOutlet weak var legPressPB: UITextField!
    @IBOutlet weak var pullUpsPB: UITextField!
    
    var textFields =     var textFieldKeys = [       "benchPressPB",       "squatPB",       "deadliftPB",       "ohpPB",       "rackPullPB",       "legPressPB",       "pullUpsPB"     ]
    var textFieldStrings =
        
        override func viewDidLoad() {
            super.viewDidLoad()
            self.benchPressPB.delegate = self
            self.squatPB.delegate = self
            self.deadliftPB.delegate = self
            self.ohpPB.delegate = self
            self.rackPullPB.delegate = self
            self.legPressPB.delegate = self
            self.pullUpsPB.delegate = self
            textFields = [benchPressPB, squatPB, deadliftPB, ohpPB, rackPullPB, legPressPB, pullUpsPB]
            for (index, key) in textFieldKeys.enumerated() {
                let aValue = UserDefaults.standard.string(forKey: key)
                textFields[index].text = aValue
                textFieldStrings.append(aValue ?? "")
                
            }
        }
    
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        let newText = textField.text
        if let index = textFields.firstIndex(of: textField) {
            textFieldStrings[index] = newText
            UserDefaults.standard.set(newText, forKey: textFieldKeys[index])
            
        } 
      return true
    }
}
// My full code for my app, to provide a new default value, do I first need to declare it as a variable? or am I completely missing the point. Apologies if this is a completely dumb question. I am literally brand new to this and was advised the best way to learn was to just build something instead of a boring udemy course. I had to delete some of the code to post... UITextFields() & UIString() Really appreciate all your help! 

There is something that doesn't mean anything:

 var textFields =   var textFieldKeys = [    "benchPressPB",    "squatPB",    "deadliftPB",    "ohpPB",    "rackPullPB",    "legPressPB",    "pullUpsPB"   ]   
var textFieldStrings =

The error here:

        if let index = textFields.firstIndex(of: textField) {
            textFieldStrings[index] = newText
            UserDefaults.standard.set(newText, forKey: textFieldKeys[index])
            
        } 

textField is UITextField ; textFields is an array of String (I assume) So you should search for newText. But it is an optional, you need to unwrap first

        if let newText = textField.text, let index = textFields.firstIndex(of: newText) {

Hey, thanks for the help! I've been trying to work this one out for a few days still. So do I need to delete the error here part and replace with the unwrapped line of code? I'm a little confused still.

I tried declaring the newText as a variable it got rid of the editor placeholder in source file error but I was met with plenty more. Sorry if this is very basic. Appreciate the help!

First point is to solve this:

 var textFields =   var textFieldKeys = [    "benchPressPB",    "squatPB",    "deadliftPB",    "ohpPB",    "rackPullPB",    "legPressPB",    "pullUpsPB"   ]   
var textFieldStrings =

What is your intent ? Where did you copy it from (with errors) ?

Do you mean :

var textFields :  [UITextField] = []
var textFieldKeys = [ "benchPressPB",    "squatPB",    "deadliftPB",    "ohpPB",    "rackPullPB",    "legPressPB",    "pullUpsPB" ] 
var textFieldStrings : [String] = []

If so, your code is now:

class ViewController: UIViewController, UITextFieldDelegate {
    @IBOutlet weak var benchPressPB: UITextField!
    @IBOutlet weak var squatPB: UITextField!
    @IBOutlet weak var deadliftPB: UITextField!
    @IBOutlet weak var ohpPB: UITextField!
    @IBOutlet weak var rackPullPB: UITextField!
    @IBOutlet weak var legPressPB: UITextField!
    @IBOutlet weak var pullUpsPB: UITextField!
    
    var textFields :  [UITextField] = []
    var textFieldKeys = ["benchPressPB", "squatPB", "deadliftPB", "ohpPB", "rackPullPB", "legPressPB", "pullUpsPB"]
    var textFieldStrings : [String] = []
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.benchPressPB.delegate = self
        self.squatPB.delegate = self
        self.deadliftPB.delegate = self
        self.ohpPB.delegate = self
        self.rackPullPB.delegate = self
        self.legPressPB.delegate = self
        self.pullUpsPB.delegate = self
        textFields = [benchPressPB, squatPB, deadliftPB, ohpPB, rackPullPB, legPressPB, pullUpsPB]
        for (index, key) in textFieldKeys.enumerated() {
            let aValue = UserDefaults.standard.string(forKey: key)
            textFields[index].text = aValue
            textFieldStrings.append(aValue ?? "")
            
        }
    }
    
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        let newText = textField.text ?? ""      // textField.text is an optional, but textFieldStrings[index] is String, not String? ; so ?? make it sure you get a String (maybe empty) and not String?
        if let index = textFields.firstIndex(of: textField) {
            textFieldStrings[index] = newText
            UserDefaults.standard.set(newText, forKey: textFieldKeys[index])
        }
        return true
    }
}

Try with those changes and tell what you get.

Thanks Claude, you've been a great help. I can sorta see how this works now with my very limited knowledge. I did have code after var TextFieldsStrings = (I think [string() though but it wouldn't let me send the code in here. (domain error)

May I ask how you learnt swift? and if you have any advice?

You probably had written

var TextFieldsStrings = [String] ()    // without the space

and got the error:

Please fix the following URL related errors. This domain "" is not a permitted domain on this forums.

This is a forum bug ! I filed a bug report on it.

So, does your code work now (or at least compile) ? If so, you should close the thread (just mark my previous answer as correct answer).

As for learning Swift, I think I did like a lot of us:

• learn Xcode and iOS API (I used books that are now totally outdated)
• work from books to practice Swift (Apple Books are pretty good)
• used on line materials (Stanford course for iOS notably).
• and then start developing some app, making extensive use of this forum and other information on the web each time I had a problem.
• and most important, Kept continuing learning over the years…

Wish you good learning and enjoy.

Thanks Claude - that's exactly what happened.

yes my code complies now but I have a new set of problems, which I'm going to try tackle myself over the next week. I will take your advice on how to manage and learn how to approach these.

Of course, I'll mark the answer correct now and you too, have a nice day! Appreciate your help ALOT!