What do I say about why I left my last job?, bank teller here

They always ask why I left, and I freeze. This is my first time dealing with any of this. I also do not want to seem like a risk. What do you say?

Say it plainly and move on to what you can do. Interviewers usually move on if you do.

Practise it out loud with a friend. Mine felt natural after the third try.