My VFD/inverter is showing an er.04 er 04 on the display when the spindle starts spinning

The Er.04 (if you are using the YL VFD), generally refers to the ramping up and ramping down of the spindle, but it can be a result of a short circuit between the VFD and the spindle. You can increase the time the spindle accelerates (ramps up to the set rpm/frequency). Additional Information: