If it will not spin it is probably not the fault of the agitator, so you don't have to do anything about it.

Take off the front cover and see if there are blinking lights on the motor. If so, these are error codes which you can interpret here.

HydroWave GE Washer Repair Guide

The first thing you want to do is to reset it, sometimes that is all that is needed. My uninformed guess is that the mode shifter may have to be replaced.