When I add PID:

-- CALC.RAFIG
return pid("IAC_LTD_DMA")+pid("IAC_STD_DMA");

I can't find the underlying PIDs to create the calculation. What could I be doing wrong?

Idle Airflow Correction AC Off, In Gear {CALC.RAFIG}
Expression:
Grams/s = {GM.IAC_LTD_DMA.gps}+{GM.IAC_STD_DMA.gps}
is NOT valid because:
Required parameter {GM.IAC_STD_DMA.gps}, at position 42 is not selected.
PID value cannot be determined because
the following error would occur:
Expression not valid: Required parameter {GM.IAC_STD_DMA.gps}, at position 42 is not selected.
Idle Airflow Correction AC Off, In Gear {CALC.RAFIG}
Expression:
Lbs/Min = {GM.IAC_LTD_DMA.lbpm}+{GM.IAC_STD_DMA.lbpm}
is NOT valid because:
PID value cannot be determined because
the following error would occur:
Expression not compiled