On the place where you're supposed to pull the lever. It says its locked and I can't pick it

First check out the rooms to the north of the lever, then the ones to the south. Search for keys and also the amulet. The entrances are a bit hard to find, because it's so dark over there. Just keep looking and you'll find them.

Petrarcus ap Bhaal.

Yeah you need the paw amulet, and keys. Try going into the options and turning up the brightness, so you can see the doors. Enter with caution there are traps to deal with.