I did contact with Apple support and this was the answer:
This is Elizabeth, and I'm an Advisor with Apple Developer Program Support. I am following up with you regarding your request for help with creating a sandbox tester in App Store Connect but getting an error message.
After further review, it appears you have used your personal Apple ID "” as the rescue email for the sandbox tester Apple ID which makes it ineligible.
You can properly set up your Sandbox Apple ID for testing in-app purchases using the steps in the “Create a sandbox tester account" section of App Store Connect Help.
If the Apple ID was ever used on the iTunes Store or App Store, the email address and any sub-address alias that contains a "+" character isn't eligible as a Sandbox Apple ID.
IMPORTANT: Entering your Sandbox Apple ID information in the iTunes & App Store section of the Settings app can invalidate your Sandbox Apple ID.
To use your app with your Sandbox Apple ID, build and run your app in Xcode.
When testing on iOS or iPadOS devices, sandbox accounts are stored separately, and you can control your sandbox account in Settings on the device. Simply build and run your app in Xcode. The non-test Apple ID isn't affected.
For macOS, the sandbox account appears in App Store preferences after you use the device to make an in-app purchase the first time. Open App Store, choose App Store > Preferences, and then click the Sign In button to sign in using a Sandbox Apple ID. When you're done testing, sign out of the Sandbox Apple ID in App Store preferences.
Note: If your test device has macOS 11.1 or earlier installed, sign out of the Mac App Store, then build your app in Xcode and launch it in the Finder.
Now you can test your in-app purchase implementation without incurring transaction fees.
Each in-app purchase Sandbox Apple ID must be a new and unique Apple account. You can’t reuse existing Apple accounts to set up Sandbox Apple IDs for in-app purchases.
Explore developer resources to get answers to frequently asked questions, watch video tutorials, stay up to date with the latest news, and more.
ggirotto
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Same problem here
I have the same issue here... It's very annoying
Hi Eskimo. I'm targeting macOS platform.
Same question here...
Did you achieved debugging? My System Extension freezes on "Waiting to Attach"
Did you solved this issue?