using "IF" with "DLookup" in ms access - vba

Question

I am making a form in MS Access, my version is 2019 from Office 2019. I have problem with duplicate entry of the invoice for purchased merchandise. Each supplier has their own way of defining their "invoice numbers" and sometimes they are using similar numbers since some use "date" to define their invoice number for example , company A invoice no is 202107001 and Company B also has 202107001 since it is July of 2021 and it happens to be the "first bill" of their month... some supplier do not have "invoice no." because they are individual selling handmade product.
Therefore, I have to write vba code to prevent entering "duplicate" invoice number from our suppliers! Here is my unworking code...
not even it doesn't work but also shows "method or data not found" too! Would there be anyone kindly help please?
Here is the condition I intend to do with the below code
"If the entered supplier invoice number is a duplicate value according to their own identification scheme, the message box should appear as the latter part of the vba code..."
Private Sub txtIMHead_supinvoicenumber_AfterUpdate()
Dim supinvoicenumberstring As String
supinvoicenumberstring = Me.IMHead_supinvoicenumber.Value
If Me.txtHead_supinvoicenumber = DLookup("[IMHead_supinvoicenumber]", "tbl_IMHead", txt_IMHead_supid) Then
MsgBox "This supplier's invoice number" & supinvoicenumberstring & ",has been recorded!" _
& vbCr & " Please stop the process and record another invoice!" & vbCr & "Thank you very much!"
Me.Undo
End If
End Sub

Answer 1

WHERE CONDITION argument needs to specify field that criteria applies to:
DLookup("[IMHead_supinvoicenumber]", "tbl_IMHead", "some field name here=" & Me.txt_IMHead_supid)
However, that expression will return IMHead_supinvoicenumber from first record that matches the given supid. You need to know if invoice number already exists for selected supid so must use invoice number in criteria as well.
If Not IsNull(DLookup("[IMHead_supinvoicenumber]", "tbl_IMHead", _
"some field name here=" & Me.txt_IMHead_supid & _
" AND IMHead_supinvoicenumber='" & supinvoicenumberstring & "'")) Then
An option is DCount:
If DCount("*", "tbl_IMHead", "some field name here=" & Me.txt_IMHead_supid & _
" AND IMHead_supinvoicenumber='" & supinvoicenumberstring & "'") > 0 Then