IIf formula in microsoft project custom field

Dale_HowardMVP Yes, I copied your formula and also tried typing it in and picking the fields & operators, but it gives me an error message (see screenshot attached).

The difficulty is that it does not indicate where the error is, I tried if a simple IIf formula works, but not even that I can get to work.

So far only the formula [Remaining Work]/[Remaining Duration] has worked on its own (as a number type).

With the IIf formula I want to achieve that the project managers see when a task is complete, so they don't need to pay attention to these tasks when they assess how much work still is to be done in the remaining time for a task. So it does not necessarily need to show "n/a" it could also show something else (like a number or a flag)