I think that is in the engine module.
You need to activate the engine module in your modules.ini (amxmodx/configs/modules.ini)
change:
Code:
; ----------------------------------------------------
; Engine - provides engine functions core to Half-Life
; ----------------------------------------------------
;engine_amxx_i386.so
;engine_amxx.dll
;engine_amxx_amd64.so
to
Code:
; ----------------------------------------------------
; Engine - provides engine functions core to Half-Life
; ----------------------------------------------------
engine_amxx_i386.so
engine_amxx.dll
engine_amxx_amd64.so