FIX sandbox new user creation what a WASTE

Is it a question expecting answer ? If so, you should formulate the question more precisely.

Or is it just to express some frustration at the situation ?

If so, this forum is not the right place as other developers can't do anything for you.

You'd better either contact Apple support or file a bug.

Good luck.

Yes, it's complicated. And often something is set incorrectly like the signature of the app. If you don't delete the app before reinstalling from Xcode then the system will think you are trying to create a 'real' user not a 'test' user. The good news is that once you have a good test user you can reuse that user over and over again.

So I just tried it and it was quite easy.

App Store Connect

Users and access

Under Users/Sandbox/testers

Click "+"

Enter all information and then "Invite".

I used my email address with an added number: myEmailAddress123@gmail.com

I used an ordinary password, nothing special

Which step above is tripping you up? Is it possible that you are using an email address that has already been used?