I'm assuming there is a start lockout if the PTO is engaged?
Could be PTO is engaged, or a bad switch or open wire, or CPU (if it's got one), etc. Maybe try bypassing the PTO safety switch?
On my new B3350, one of the first things I did was bypass the seat occupied switch (it's normally open, so requires a jumper wire to bypass). Not recommending bypassing safety stuff, but in my case I have a difficult time looking aft without scooting forward on the seat to turn by bod - - and that kills the engine - - obviously not workable.