you can have a bad circuit in the card itself too
seen that happen a few times with 24 in 1 direction

the best bet would be to have a forklift technician look at it
these systems can be dangerous if you dont know what youre looking at
worst case scenario, the truck runs away and plows through a wall/ merchandise /people if you do something wrong in your repair
in that case you would be held personally liable

Code 24 is usually a contactor. However, usually is won't go just one direction when you get a code 24. Is the contactor pulling in at all? Check all your fuses and for loose connections. Usually when you get a code 24, you can see burnt contact tips, or you have a contactor that is mechanically binding and wont pull.
If you have a helper, block up the truck so the drive tire is off the floor and have the helper jiggle the contact tips while you try to travel. If that makes the drive tire turn, it is a contactor problem.

Another possibility is that someone has worked on the truck and hooked up some power cables incorrectly.