Error checking

Maybe like this:

Private Sub Itemnum_AfterUpdate()
    Dim val         As String
    
    With userforms1
        val = .Itemnum.Value
        If IsError(Application.Match(val, Range("price").Columns(1), 0)) Then
            MsgBox ("Invalid Item Number!")
            .Itemnum.SetFocus
            Exit Sub
        Else
            .desc.Value = WorksheetFunction.VLookup(val, Range("price"), 2, False)
        End If
        
        ' ...
    End With
End Sub