If page 40 doesn’t help in the operating manual then you need to follow the linkage from the cruise control end to the hst pedal end looking for damaged parts or missing hardware. This will also include a checkout of the brake system since depressing both brake pedals simultaneously disengages the cruise control.
A WSM would be best for troubleshooting.
If you’re missing the operating manual you can use the myKubota app and view it or download it for printing.