well with all electric trucks checking the battery and making sure it is in good condition and charged is the first thing.
But as techtrainer said there are many things that can cause this problem.
check your fuses (fu8) specifically.
If you remove the panel behind the operator it will expose the control panel and contactor panel. There are some large fuses on the panel connected to the buss bars, there is a 150A fuse connecting the B pos to field windings and auxiliary motor, you'll need to check this and make sure it is not open. You may as well check the other fuses too but FU8 is the fuse described in the troubleshooting manual for this code.

There are also cables to the field windings and field circuit to the HCM (H Bridge Control)
TM1 and TM2 could be bad
VM could be defective
Drive motor may have open windings

You will need the troubleshooting guide to get the info you need to troubleshoot this problem effectively.
But start with checking battery and fuses and connections on cables etc.