The dropdown you are showing is showing the attributes of the entity TeamMember. $CountForUserList is not one of them.
Using the expression wizard’s variable dropdown you will not see the $CountForUserList in that dropdown since it only shows objects, not variables. The text ‘Variable’ there is wrong and confusing.
Not using the expression wizard you can type $CountForUserList in the textarea by yourself. It will get recognized when you type ‘Cou’ and press Ctrl+space.