It should be on the backside of the hydraulic control valve that you plugged the hoses into.That could be part of the problem. There is no lockout valve on my tractor. I was told (and have read several times) that I don’t need one. I see no leaks indicating a bad cylinder. If I need a lockout valve, then I see that as a simple fix.
I’ve never heard of it not being required on an h2 with a plow.