Insert Google OAuth 2.0 token into nextcloud


I’m trying to implement external access to nextcloud via Google account. I followed some Google tutorials and some other tutorial I found in this forum to implement SAML. I’m now facing an issue because I can’t log in with any user, even if it is already registered on nextcloud GUI; also the account is being authenticated by Google, but it’s raising an error “403: app_not_configured_for_user”.
I think I’m missing something which I guess would be a Google token to authorize the access. Does someone has an idea of what might be happening? Or maybe how to include Google token into code?
Thanks in advance.