Hi epalf,

When playing power chords, you have to make sure your fingers aren't getting in their own way. Power chords are also essentially barre chords, so you have to make sure your grip is strong enough to hold the strings down.

I suggest focusing first on the 3-finger power chords, and then working your way up. Also, it's completely okay to skip forward in a lesson if you're stuck. If you keep practicing the same lesson, you may get bored, or frustrated and that's no fun at all.

Hope that helps~