I would assume you hare using early binding and ave a reference to the Word Object module set in your project. I used late binding as shown and it worked without issue from Access 2016:
Code:
Sub Textbox()
Dim wdApp As Object
Dim wdDoc As Object
Dim wdShp As Object
Set wdApp = CreateObject("Word.Application")
With wdApp
.Visible = True
.ScreenUpdating = False
Set wdDoc = .Documents.Add
With wdDoc
Set wdShp = .Shapes.AddTextbox(Orientation:=5, Left:=10, Top:=10, Width:=10, Height:=10)
End With
.ScreenUpdating = True
End With
Set wdDoc = Nothing: Set wdApp = Nothing: Set wdShp = Nothing
End Sub