Remove the line "include devpac:system.gs" and insert the following lines, and it will assemble:
include exec/exec_lib.i
include exec/execbase.i
include exec/memory.i
include dos/dos.i
include devices/trackdisk.i
include devices/timer_lib.i
include resources/disk.i
include resources/disk_lib.i
include hardware/custom.i
CALLSYS macro
jsr _LVO\1(a6)
endm