Yes you can. In Magic the Gathering, it doesn't matter if a spell/ability will 'do nothing' on its own resolution. You're free to cast it if all you want it to do is trigger something else, for example. Just as long as the spell/ability is not a targeted one, as they must be able to select valid targets or else you are not allowed to cast/activate it.
So yes, you can use Estrid the Masked's +2 ability just to put counters on her, even if you have no enchanted permanents, or you do but none are tapped.