Update your Mac to a newer version of Catalina. On your Mac go to System Preferences > Software Update to update.

Does the article at the following link answer your question? swiftdevjournal.com/xcode-11-missing-view-controllers/ If not, update your question with how you created your Xcode project.

The 11.2 GB package is a compressed file. You need enough disk space to store both the compressed and expanded files. A good rule of thumb is you need 5 times the space of the package you're downloading, which would be 56 GB in your case. See the article at the following link for more information: meandmark.com/blog/2020/04/disk-space-required-to-install-xcode/

The Xcode 12.0 GM seed does not include the macOS 11 SDK, which the new SwiftUI app cycle requires for Mac apps. You will have to install the Xcode 12.2 beta and use that until macOS 11 ships.

In Xcode's Source Control preferences, click the Git button. There is an Ignored Files list where you can add files to ignore. One thing to keep in mind is if git is tracking a file before you add files to ignore, git will keep tracking the file. You will have to temporarily move the file out of the .git folder (This basically involves moving the file out of the project folder), commit to git, then move the file back.

Xcode 12 requires macOS 10.15 or later. It is not supported on macOS 10.13.6

A split view has multiple view controllers: one for the split view and one for each split view item. My guess is one of the other view controllers is getting the message to change the background color. Which view controller has the button that triggers the IBAction? If you want the whole window's background color to change, you should put the IBAction to change the color in the split view controller. Then you can go through each child view controller and change the view's background color.

Choose App as your project type. Choose Storyboard from the User Interface menu to get a view controller file to follow along with the tutorial. Read the article at the following URL for more detailed information: swiftdevjournal.com/xcode-11-missing-view-controllers/

Choose App. In Xcode 12 Apple consolidated a bunch of iOS app project templates into one template: App.

I see no reason why you couldn't download Xcode 12 to an external drive, expand the xip file on the external drive, then copy Xcode from the external drive to the MacBook Air.

You have to create a command-line tool project in Xcode in order to run a C program. For small C programs, you are better off using a text editor like TextMate, BBEdit, or Visual Studio Code. I know TextMate has a bundle that lets you run C programs from the editor.

List the steps you took to create the project and add the files to it. What type of project did you create? If you create a new project of a similar type and Xcode and run it, do you run into similar issues?

Third party frameworks should be installed in /Library/Frameworks. The System folder is for Apple's frameworks. How are you adding the framework to the project? Is the framework in the Frameworks, Libraries, and Embedded Content section of your app target? If not, what happens when you click the Add button and add the framework that way?

Xcode 12.0 does not include the macOS 11 SDK, which the multiplatform SwiftUI templates require. Install the Xcode 12.2 beta to get the SwiftUI templates. The following article has more detailed information meandmark.com/blog/2020/10/where-did-the-multiplatform-swiftui-project-templates-go-in-xcode-12-0/

Does the following post answer your question? Xcode new Mulitplatform project: only Game app choosable - https://developer.apple.com/forums/thread/664039/