I've pulled the belt to make sure a pulley isn't seized up. It seems a little hard to pull but not terrible or doesn't seem stuck. It runs If I'm not in the seat.
Possibly a safety switch. I don't know what your model looks like exactly, but on my Rogue there are switches on the control levers, the parking brake and the seat. If one of them is disconnected or faulty it may not let the mower PTO run.
Check the battery first. If you have load tester would be helpful. Pto puts a load on battery’s so if your battery is not strong to handle the pto but can be strong enough to start.
