Husqvarna mtz61 dies when parking brake released.

Seat must be occupied if brake off and PTO on.

See wiring diagram
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/husqvarna/ride-mower-turf-care/z-4217-bia-968999280-2005-08/wiring-schematic

Check the engine ground wire. It may only be obtaining ground from the brake switch.

You have a safety switch not working. First check your seat switch. Try to engage the PTO with the parking brake on. Blades should start but die when brake is released. My guess, seat switch. Check it is completely connected, these can be hard to connect. Make sure you hear a "click" when the connector is pressed into the switch.

your deck and clutch safety switches are bad,replace both, Husqvarna Safety Switches

your manual, Husqvarna RZ4621 BF Operator Manual

read page 49, http://www.mowersdirect.com/manuals/RZ4621_manual.pdf

This sounds like the seat switch. Keep in mind newer model seat switches cannot be jumped or disconnected.

check the safety switches on each lever

cut the wires running to the switch...try again...if same, stripp back wires, and twist together...wrap in tape or use some sort of connector...it's electric and should be treated as such...try again...still ?? Start finding those switches and repeat the process til it works...theres usually 3 or 4...

well the not moveing could be the brakes are rusted look were the break rod goes put some wd40 on that its on the side of the trans and you will need to move back and forth to free it up and the cluch break switch is bad i hope this helps you

First check the seat switch,make sure it is plugged in tightly, these plugs have a safety tab built into them that will cut the engine off if you unhook it,sometimes the plug latch pulls out slightly and this will cut the engine, There are two other switches in the system one on the foot clutch and on on the mower clutch, if you follow the white wire from you ignition switch it will lead you to both of them,check those connections also.

the seat switch is the problem. the switch itself could be bad, but usually theproblem is that the connector is just not plugged in all the way [by the way, trying to bypass the switch by inserting the wire doesn't work because there is a "switch" inside the connector to prevent you from doing just what you did]