We don't have the option to sign in into your Make account with Google SSO at the moment, which will Make consider automatically in case you've logged into iClosed with Google Single Sign On.
It is something that is going to be available in the future and until we have it available, please use following workaround to have Make sign in from iClosed.
Log out of your iClosed account
Enter user email and confirm Password reset, to receive Password reset link
Open link and set password manually
Sign back in iClosed with user email and password and navigate to
Integrations to connect your Make manually
Learn more about Make integration here.