Error Message: "The email address you entered already belongs to an existing Apple account". How to have an email re-available for Sandbox testers?

I accidentally added the tester email in the bad section in the iTunes Connect. There is two options:

The one I choose by mistake is the User iTunes Connect, but I had to choose the Testers Sandbox.

What I want now is simply to add the tester in the good section, but I have an error message that says:"That email address is already associated to an existing Apple id."

How to get an email available to sandbox testers in iTunes Connect?

(I check that ticket, but it seems not to have solutions)