"I can sleep on the sofa."
Translation:Mogę spać na sofie.
No, that's not "musieć", "mogę" is a form of "móc".
"móc" (mogę) can mean that I am allowed to do something, that I am able to do something, etc.
"umieć" means that I know how to do something (and that's why I can do it). For example "Umiem spać na sofie" is possible, but rather unusual, and I guess it rather means that you are able to fall asleep on a sofa even though it's not the most comfortable place to sleep.