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 for G.A. Suite with an Email authentication option instead of Office 365. You are welcome to find information on how to use Email authentication at the following link.
Please note that, in order to have access to G.A. Suite, new users would have to log in with the Email authentication option as well.
2. G.A. Suite has a published Enterprise App that can be added through the Entra ID (formerly Azure AD).
Follow the steps described in this article to add the G.A. Suite app to the Entra ID (formerly Azure AD).
3. 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 the Microsoft 365 tenant to whom the administrator granted permissions will be allowed to log into the G.A. Suite account through Office 365 authentication.