This is referring to a field on the Part (part master) page.

I assume you are starting the new part process at the Inventory Part page, which will create the record at the Part page, but the system wants a value for this mandatory field (which is not a standard mandatory field, but likely a configuration in your system).

This “Part Main Group” field is here:

And as Matt has already mentioned, in the standard this field is not a mandatory field. Therefore, this was configured in a customised way.