Multi Select Combo box Attribute / Condition on GI

Hi,

In the case we have a multi select combo box attribute like this :

I want to isolate a specific value of the combo box like the last one (ValueID=Complété).

For this, i put a formula in the GI and I get this error message(Picture Below).

=IIF(INSTR([FSServiceOrder.DOCUMENTS_Attributes],'Complété')=1,True,False)

I putted a Schema Field “FSAppointment.Completed” 

Do you have an idea how to resolve this?

Thanks!