Just finished charging battery and while fastening the screw of the battery upper frame i noticed it keeps turning and not tightening the frame to the body. Am i missing a washer here?

What car, what year?

Datsun 240z

If it is  not tightening, it is possible the threads are stripped either on the bolt or the captive nut welded to the back of the firewall. Or... possibly you have an undersize bolt. The one in your pic is not OE, it should look like this one. Also, the bracket tab usually has a rubber insulator.

Thanks. Definitely not the bolt i have.