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. 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).
2. The Microsoft 365 tenant administrator should sign up to G.A. Suite using Microsoft 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 Microsoft 365 authentication.