Can't Subscribe == Invalid Username or Password

I have tried to subscribe a VAIO machine that I installed Red Hat, CentOS and Ubuntu but it keep saying "invalid username or password".
I have used all flags from the subscription-manager module but still couldn't either subscribe using new credentials or the old.

Can only access RHEL via CLI but wanted to install GNOME for exam practices.
Keep nudging for "repolist" which I can't install any repo because of subscription.
Please what step should I take to remedy this situation?

Thank you!