How to create a right validation rule in MS Access

I tried to create the following validation rule, but it doesn't work.
I want to look up the inactive date value in the Vehicles table after the user entering the vehicle_no in the current form.
if the inactive date is not null, an error message will appear.
no problem otherwise.
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Don't you need to use a IIF function!?
mawinghoAuthor Commented:
why? it should be true, right?
Can you post here the text that you have on your validation rule please? It's hard to get it from the picture...
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It's difficult to be 100% sure without knowing the control/field names - can you post a cut-down sample database?
The syntax of your validation statement is incorrect.
IsNull(DLookup("[inactive_date]", "Vehicles", "[vehicle_no]" =  & [vehicle_no]))
To this:
IsNull(DLookup("[inactive_date]", "Vehicles", "[vehicle_no] = " & [vehicle_no]))
mawinghoAuthor Commented:
The correct validation rule should be
(DLookUp("[inactive_date]","Vehicles","[vehicle_no] = '" & [vehicle_no] & "'")) Is Null
I want to know this validation rule is it only apply on this field when adding a new record?

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