You didn't read the part about using the FillBM function? Use the following FillBM function which doesn't throw an error if the bookmark is missing.
You shouldn't need to update the fields unless there are cross references toi the bookmark.
Private Sub CommandButton1_Click()
FillBM "Test", Me.TextBox1.Text
End Sub
Private Sub FillBM(strBMName As String, strValue As String)
'Graham Mayor
Dim oRng As Range
With ActiveDocument
On Error GoTo lbl_Exit
Set oRng = .Bookmarks(strBMName).Range
oRng.Text = strValue
oRng.Bookmarks.Add strBMName
End With
lbl_Exit:
Set oRng = Nothing
Exit Sub
End Sub