Unable to add Tester Email Address to Google Play Console

I am unable to add new Testers Email, in Google play Console. In my Application , I have a list called Beta Review Team ,Under that I have a list of emails for test users. It has worked for the past few years. I have Currently 84 testers. (Find screenshot 2)

I need to add one new email to this list. Is there any limitations for test users? Can anyone please help me?

Press ENTER after you filled the emails to the textbox. I discovered it after I got emotional ("do you want an Enter little **** or what?")

I was getting an error ("This email address doesn't exist") when trying to add a new email to my email list.

I figured out that I can only add as tester an email that was previously added to my Google Developer account under "Users and permissions". So after I added that email there and the person accepts the invitation to join my account, then I was able to add it with success to my test email list.

When adding an email address to a tester list, one limitation is the email must be linked to a Google account. This is easy to forget when adding co-workers that have corporate email addresses.