So, after downloading once more new Apple Certificates and creating a new Tester (myself) I was happy when after delivering the app to the Store, Transporter did not warn that I could not testflight the app.
Yet, in the TestFlight section of the Store, it says that the app is not eligible for TestFlight. In fact I came to know that TestFlight App is available for Monterey only.
Therefore, since my MBP is using BigSur and cant upgrade to Monterey, BigSur users are cat off from TestFlight.
It also means that BigSur users cannot test the apps to be delivered to the Store as they used to do, because the old procedure (launching the app and getting the prompt to enter login & pw) returns a message that the app is corrupt.
Any other way to pre-test the apps without buying a new Mac?
BTW: I do not need any other pre-tester; just myself.