the black knurled **** would be the manual lowering valve and generally you would turn it counter clockwise to open the valve or maybe pull on it.
What i would be checking for is the electric solenoid valve in the lowering manifold block which is usually located in the same place the manual valve is.
If you have checked the lowering switch circuit and it is turning on and off correctly and you verify this is not an electrical problem then go to the electric solenoid valve on that manifold block and remove it and check it to see if it is stuck open or has debis in it. If there is not a separate manifold block like what i speak of then it may be located on the pump assy. Being an orderpicker i'd bet it has a separate block for lowering control.

Some of these cartridges have a small screen on the end covering the openings and sometimes that will come apart and the metal debris from that can cause the valve to hang open.

Note: when removing the solenoid coil and cartridge valve make sure the platform is fully lowered and be prepared to catch some oil as it may start to gravity feed oil from the tank and that can wind up being a huge mess if you do not have anything under it to catch the oil. One way to insure it does not gravity feed from the tank is to pinch off the return line with some vise grips, just be careful not to damage the rubber hose by squeezing it too much.
And most importantly if you are unsure of any of this please call a tech in to do it. ;o)

No, that is abnormal.
Was the unit powered off at the time? If not try when battery is unplugged.

I looked up the owners manual for your truck and it does have a lower switch. I have never tried using the emergency lower while the truck was requesting a lower command, there is a possibility you are correct and the lower switch is stuck causing the solenoid to remain engaged which may make the **** difficult to use this is why I'm now asking to try with no power connected.

If it is the same with no power, MY next step would be removing the valve and inspect/replace.
If you don't have experience with hydraulics you may want to hire someone who is. I wouldn't want to see anyone get hurt, there is potential for excess of 2500psi to be in the system.

Thank you so much for the advice. I found the valve, it has a black plastic knurled ****. I tryed to push it in but the pressure is unbelievable, I was able to push it in a little bit and turn it but it wouldn't stay. Do you think that is normal?

Thanks

Look for an aluminum valve block behind the drive motor. One or two of the valves will have a small knurled **** sticking out, push it in and turn 1/4 turn to lock in.

It may be located somewhere else but it will be in the tractor compartment in a valve block. This is probaly your issue or as you said a faulty lower switch if it doesn't have variable lift (single speed lift/lower) if it is variable there will be a pot for lift and lower and no switch so that wouldn't be the cause.

Hi, thanks for your reply. Yes it is going up a lot slower. Do you know where the emergency lower valve might be located?

I'm not too familiar with your machine however I am with nos15 and other brands.
When you lift does it lift as normal or is it slow?
Check that the emergency lower valve is shut.