Welcome to JUST ANSWER small engines, let me try to help you !!
The metal guard needs to be removed, it has a couple of bolts that hold it in place.
Which pulley is the problem?
The sping loaded idler?
I has to have a removable guard to keep belt on. It has to be loosened or removed to get belt around it.
I'm using the Deere parts site for the L100
#11 is for the transmission drive belt. You are just doing the deck??
The bolt has to be loosened, it can be tight, just be sure you are turning it in the correct direction.