I am unable to add a staff as yje select username search box doesn't show the required results , it throws not found for all users

I had the same problem. It will work if you search by Name. They should fix this.

And instead of a stupid search, they should SELECT * FROM WP_USERS and allow you to select the users you want to add as staff. That would make so much more sense. But obviously these idiots don't have sense. Lazy programmers. They can't even write code to clean up the database when you delete a record.