Can I apply for Postdoctoral Research Assistant as a fresh MSc graduate?
Unfortunately I expect this is a mistake on the university's part. Many people apply for postdocs in the final year of their PhD, so perhaps they misinterpreted your CV and thought you were in that category.

There is likely to be a requirement in the formal job description that candidates must have, or be near to completing a PhD, so even if you were to attend and do well at the interview, I highly doubt they would be able to make you a job offer anyway.

My advice is to send them an email thanking them for the invitation to interview but clearly state that you do not have a PhD nor are you a current PhD student. It is likely they will rescind the interview offer.