It's not the one with TOP on it. That is the mower cutting height adjustment. To the right of that one is a slightly smaller knob that is mostly parallel to the floorboard. It has a lever looking symbol and a curved arrow. This is the PTO drop speed control. At one end of it's travel it will lock your mower deck and prevent it from raising or lowering.
Try putting this one in the middle of it's movement range. It is pretty stiff to turn, but make sure it is in the middle of it's range. The full open setting will allow your deck to fall quickly, the closed setting will not allow deck movement. Look at your owners manual on page 18, foot and hand controls, item 3.
If you are sitting in the seat and looking down between your legs it is the one to the right of the mower cutting height adjustment.
Since you have the backhoe mounted testing the three point for movement will not show you anything. You do not have your three point hitch installed.
Another thing to check is the PTO selector. Sitting at your seat, look at the levers on the left. The lever furthest to the left determines which PTO position operates. This lever should be in the rearmost position. There is a symbol of a tractor with a gear under it at that position, indicating the mower deck. This lever in the mid position operates both PTO position and if the lever is in the forward position it operates the rear PTO only. To me this is a bit confusing, but it is the standard way of Kubota controls. This is explained on page 45 of the manual.
I hope this helps. I was quite intimidated learning what all the knobs and levers did at first and I operate very large machinery for a living!