what do they call the storage bar on back of motorcyle cover

It’s usually called a sissy bar or backrest on a motorcycle. If you mean the storage bag that mounts there, people usually call it a sissy bar bag.

Common names

• Sissy bar : the upright bar on the back of the bike.
• Backrest : the padded part a passenger leans against.
• Sissy bar bag : the storage bag strapped to that area.

Simple example

A rider might say, “I added a sissy bar bag to my bike for extra storage”.

TL;DR: The storage thing on the back is usually called a sissy bar bag , and the bar itself is the sissy bar.