Elements validation formula not working

We are facing an issue where the validation formula that we have added in one of elements is not working.
The requirement is to have that if the employee has selected one incentive plan, and if he is selecting the other incentive plan he should not be able to select that other plan, receive an error or something.

We created ff for this and added in one of the element's eligiblity validation formula.

But now the issue that we are facing is that, even if the employee is starting to select of the plan, he/she is still getting that error, even when the other plan has not even been selected.