If the account you are using for G.A. Suite registration lacks permission to install third-party applications to your Office 365 tenant, the following error occurs while signing up with this account:
Please use one of the following options to resolve the issue:
1. Sign up to G.A. Suite with an Email authentication option instead of Office 365 one. You are welcome to find information on how to use Email authentication by the following link.
Please note, in order to have access to G.A. Suite, new users would have to log in with the Email authentication option as well.
2. Office 365 tenant administrator should sign up to G.A. Suite using Office 365 authentication.
It is required for the tenant administrator to log into G.A. Suite and share its PWA permissions as per the article.
After this users within Office 365 tenant who the administrator granted permissions to, will be allowed to log into the G.A. Suite account through Office 365 authentication.