What should I respond in an interview?

Hi everyone. Recently I was fired from a job because the client company I was working with deemed that I was not a good fit for them, the contracting company and I ended in good terms and they assured me that my performance was good and the issue was just on the other end. The problem is, since I am currently going through interviews for a new position they ask me why I left my job and I don’t know how to answer because I was adviced against saying that I was fired because they didnt think I was a good fit. I’m still pretty new in the job world so I would really appreciate if someone with more experience could give me some advice.