usually that would be either no one is in the seat, or the seat switch is bad or the wiring to the seat switch is bad. seat switch or deadman switch if a stand up unit, but usually an "operator out of position" error, since the first thing that has to happen is the operator has to be in the proper position.
does your operator have a large wallet in their back pocket, and is a smaller person (less than 60 kilos?) ? Sometimes this causes a situation where the seat switch does not operate correctly. have them remove the wallet form their back pocket.