How to validate checkbox

Hi experts,

My requirement is to validate checkbox in Problem management.Below is the condition which I gave in FC (PM.problem.closure)

not  (closure.criteria in $file=true) and I have even tried with this condition as well. (closure.criteria in $file=false) but which is not working.

Note: Requirement is to display an error message if the checkbox is unchecked.

I am using SM9.40. Thanks in advance.