Look at the criteria of your DLookup:
    IsPresent = DLookup("SkuNumber", "PartsCatalogueT", "SkuNumber=""" & PartName & """")

Why is the criteria comparing SkuNumber to PartName? They sound like two different fields.

Ok, I changed the name from PartName to SkuNumber. I guess i was confusing it. Then I changed the DLookup like you have it.
IsPresent = DLookup("SkuNumber", "PartsCatalogueT", "SkuNumber =""" & SkuNumber & """")
However it is still setting focus on "description"
I know the Dlookup is working right because if i msgbox "IsPresent" if tells me that i am finding a match.

Is it description or Description?

What type of field is IsPresent?
In your code it is treated as text, but below the code you say "IsPresent" = True.

Also, I would wrap the DLookup in the NZ Function.
If you remove the errorhandling you are probably getting an Invalid Use of Null error.

Try this:

Dim ID AS Long

ID = Nz(DLookup("PartID", "PartsCatalogueT", "SkuNumber=""" & SkuNumber & """"), 0)

If ID = 0 Then
    Description.SetFocus
Else
    MsgBox "WARNING! This Part Number already exist!"
    PartName.SetFocus
    PartName = ""
End If

Here is the Table names and the form. I did your suggestion and it is still jumping to Description (and yes, it is Description, Not description. I had the control named PartName even though the table name was SkuNumber, but i changed it back to SkuNumber. I have noticed though, that the curser seems to stop at SkuNumber for just a split second before going to Description as though something is causing it to end up at Description, but I can't imagine what that would be.
So, I did this:

Details

Private Sub SkuNumber_AfterUpdate()
     ID = Nz(DLookup("PartsID", "PartsCatalogueT", "SkuNumber =" & "SkuNumber"), 0)
    MsgBox ID
      
        MsgBox "WARNING! This Part Number already exist!"
        SkuNumber.SetFocus
        SkuNumber = ""
End Sub
Copy

And it still jumps to Description!!

msgbox returns a 1
i figured out that the curser is stopping for a split second to clear out Skunumber. If i comment out "SkuNumber = """ it just jumps to Description.

"IsPresent" was not a field. it was a Dim Statement as in: Dim IsPresent as string.

In the Table i have a record that equals; PartsID 561 > SkuNumber = AMD 5700
So, I created a new form with only the SkuNumber Field, and here is what i get:

Somehow VBA thinks ID is 0 when msgbox says 1

Before Update

Kevin, Can you explain?

Will I think what Kevin was referring to was that you should move your code for checking the value from the SkuNumber_AfterUpdate() event to the SkuNumber_BeforeUpdate(Cancel as Integer) event.

Then you can check if ID <> 0 and give your msgbox, Clear the SkuNumber and set the focus to it and then change cancel to true and your cursors should stay in that field. In the after update event it has already started moving.

So your code might look like:

Details

Private Sub SkuNumber_BeforeUpdate(Cancel as Integer)
     ID = Nz(DLookup("PartsID", "PartsCatalogueT", "SkuNumber =""" & SkuNumber & """"), 0)

     If ID <> 0 then
          MsgBox "WARNING! This Part Number already exist!"
          SkuNumber.SetFocus
          SkuNumber = ""
          Cancel = True
     End If

End Sub
Copy

Since it appears that description is the next field in your tab order, you don't need to add code to move there as tabbing out of the SkuNumber will put you there anyway.