I mean when you run out of PP of all your moves
It's normal type. It's not limited to immunity, so can you can hit ghost types,even without foresight. Rock and steel type pokemon are not resistant to struggle.
http://bulbapedia.bulbagarden.net/wiki/Struggle_(move)
Normal it has no limit on PP and on Ghost types it can hit them without foresight or any other move like foresight.And it is regularly affected on rock and steel.