The thing is that it's doing nothing. 😄
The hope: The validation error will occur if someone chooses one of the choices in a single-select dropdown list.
I did a test field with four dropdown values. In this test, I want "C" to elicit the validation error:


I tried that with two equal signs, one equal sign, an "else" comma, no "else" comma. And when I test:

I've tried other examples listed here to validate that the general validation feature exists and works elsewhere...and they do.
When you say 'it's doing nothing' - WHERE is it doing nothing? When you try and test in Preview via the form designer? Or when you actually go to a record with that form/field attached and test there? If you're trying to see it work in Preview, then that's the actual issue. The logic doesn't fire in Preview, only on a live record. Your syntax is 100% correct, you'd be getting an error in the formula pane if it were invalid somehow.