If you add [tblFieldWorks]! and remove the quotation marks...
RechargeTo: IIf(IsNull([tblFieldWorks]![RechargeToWhoID]),"N/A",DLookUp("[ToWho]","tblChargeable",[ID]=[tblFieldWorks]![RechargeToWhoID]))
...the error is gone, but the result of the query isn't correct. Try this:
RechargeTo...