The point is not about whether you can or can't use 'poder' but in Spanish people will say You no encuentro las llaves and mean they can't find the keys. It would sound strange in English to translate this literally as I don't find the keys so we have translated it more loosely into the appropriate expression.
If you imagine looking for keys, failing, and speaking aloud to someone in the room: "I'm not finding my keys anywhere!" you can get a sense of how that communicates "unable to" without actually needing to use "can't".
Make sense?
