Add some Msgbox LargestProductNumber along the way to see what it's set to. Or step into the code and see the values. Make sure it's finding the correct starting point, and adding 1 to it.

Is ProductNumber a number or text?

Also what is the second textbox / field for? The AssignNextProductNumber.

Your code could be dumbed down a lot to be simpler
If Not IsNull(ProductNumber) Then
Msgbox "Already"
Exit Sub
End If

ProductNumber = NZ(DMAX("ProductNumber","ProductT"),0)+1

Thanks I just changed the last line and it works fine now

Oh. Guess I was too slow. LOL

That's Fine, you have added a little more information on how to start with a specific number. Thank you