Is your bar inside an auto layout frame? If so, apply Ignore auto layout option to it — this will make fixed position available

I checked, it's not on auto layout, the fixed (stay at place) option is visible but when I click that it is not getting selected, I'm unable to find the issue 

Oh, turns out i have the same problem, probably it’s a bug.

It’s time to report a bug or wait for someone from the Figma team to notice this topic.

Thank you so much for figuring that out, I’ll try to report the bug