hidden required field problem

I have a required field that shows ONLY IF a certain payment type is selected. However when I select the other payment type, checkout is not possible because of the error message which says that the required field is not filled out. How to make it required ONLY when showing? Or, make it required ONLY with that one payment method and not the other?

I have already tried to debug as follows:
– Deactivate plugins one by one checking each time.
– Switch to Storefront theme and test
– Disable any custom scripts you might have.