Log In issues for Contributer

I am a consultant helping out a non-profit with their site. They gave me a login name and password, but when I use them, I get a message that I "usually log in with a social media" account. When I do that, I can see their site, but I keep getting messages that my trial is expiring and must be extended. I don't need a personal account - I just want to log into theirs and work on the site. First time with SS. Advice appreciated.