Which application? You should get in touch with the specific developer of an app if you want to test their app in TestFlight, not fire-hosing everyone on these forums.

Okay, according to this forums issue "That is the expected behavior. In the sandbox, you can fetch in-app purchase information in the Simulator. However, you cannot purchase them. To test purchasing in-app purchases in the sandbox, run your app on a device." None of the Apple Developer Documentation pages explicitly point this out. Also, heaven forfend you supply a reasonable error message that might give us a clue as to the actual problem! Telling us about "password reuse" is about as far away from "can't do this in the Simulator" as you can get. I'm raising an FB (FB16366964) on this so that others don't waste days trying to figure out where they're going wrong.

No one else is experiencing this? I'm the only one developing an in-app purchase who's getting this error?

Provide some code. Without it, we can't really tell whether there's a bug or you've done something wrong.

You're in the wrong place. These are the Developer Forums, where developers of apps for Apple's platforms ask each other for hints and tips on coding. Your question is more of a product support one, so I'd suggest you ask it over at the Apple Support Forums. Thanks.

No one is going to download a random zip file. Post your code, properly formatted, here, and we'll have a look at it.

I don't think you can add apps in the Connect app. Just go to the website.

Just go to App Store Connect and start to add a new app you'll see what it means. It's just a product code that means something to you. It means Stock Keeping Unit. Like a clothes manufacturer has a SKU for the red t-shirt in Medium might be REDM, they'll have a SKU for the red t-shirt in Large which might be REDL.

https://developer.apple.com/help/app-store-connect/manage-subscriptions/manage-pricing-for-auto-renewable-subscriptions

Xcode > click the blue project icon at the top of the project view > click your app's name in the list of targets, then:

Create the app in App Store Connect. You don't have to fill in any details other than the name and SKU, I think.

Fixed what issue?

What exactly does your "AI-powered chat app" do? Does it offer medical advice? If so, and you don't have appropriate medical regulatory approval, then that's why you're hitting this issue. You can say that you don't provide medical services etc., but is that completely true? Does your AI bot thing offer medical advice?

Do what you like, but do it somewhere else. These are the Developer Forums, where developers of apps for Apple's platforms ask each other for hints and tips on coding, and using Apple's APIs.

It would maybe help the guys at Apple to fix this sort of thing (or at least provide a better error) if you would post your code - as Quinn asked you to.