Users and Access(Enter an email address that has never been associated with an Apple ID.)

One client has invited me as an admin of the own Apple developer's account to manage the apps.

One more client wants to add me as an admin of their Apple Developer's account but he can't do that he is getting the below message.

Enter an email address that has never been associated with an Apple ID.

Please give me a solution so that my client can add my email account as an admin to manage to Apps.