Look in the MPASM Suite library on your harddrive. There you will find a P18F4620.INC file. In this file there is a label called DEBUG, that is why you get this error. Just change the file
so you remark the line for debug, use ; to do it like the example below. This will fix it untill next time you upgrade MPLAB.

LVDCON EQU H'0FD2'
OSCCON EQU H'0FD3'
;DEBUG EQU H'0FD4'
T0CON EQU H'0FD5'
TMR0L EQU H'0FD6'