You have a syntax error,
Also, to use IIF correctly you should specify what to do when the result is true and when the result is False,
Try:
IIf([Forms]![frm_Main]![cmb_Province]="", Like "*" & [Forms]![frm_Main]![cmb_Province] ,"cmb_Province has a specific value")
I am sure you can adapt this to work in your database.
If not, then can you take a step back and explain to us what your ultimate goal is here, ...there may be a more efficient way to do this...
For eaxmple: If your goal is to show all records if no value is selected, the this is typically done simply like this in the query criteria for "ProvinceID:
Like "*" & [Forms]![frm_Main]![cmb_Province] & "*"
(No real need for IIF...)
;-)
JeffCoachman