Either of the following work calling Word from Excel. The issue is you are trying to use the Excel Application (and the Excel Selection) in a Word process:
Option Explicit
Sub ScratchMacroI()
'A basic Word macro coded by Greg Maxey, http://gregmaxey.com/word_tips.html, 9/17/2018
Dim oApp As Object
Dim oDoc As Object
Dim hyperadd As String, disp As String
hyperadd = "www.google.com"
disp = "Google"
Set oApp = CreateObject("Word.Application")
Set oDoc = oApp.Documents.Open("D:\Text.docx")
With oDoc
.Bookmarks("law").Select
.Parent.Selection.MoveRight Unit:=1, Count:=8, Extend:=1
.Parent.Selection.Hyperlinks.Add Anchor:=.Parent.Selection.Range, Address:=hyperadd, TextToDisplay:=disp
.Close -1
End With
oApp.Quit
Set oApp = Nothing
lbl_Exit:
Exit Sub
End Sub
Sub ScratchMacroII()
'A basic Word macro coded by Greg Maxey, http://gregmaxey.com/word_tips.html, 9/17/2018
Dim oApp As Object
Dim oDoc As Object
Dim hyperadd As String, disp As String
hyperadd = "www.google.com"
disp = "Google"
Set oApp = CreateObject("Word.Application")
Set oDoc = oApp.Documents.Open("D:\Text.docx")
oDoc.Bookmarks("law").Select
oApp.Selection.MoveRight Unit:=1, Count:=8, Extend:=1
oApp.Selection.Hyperlinks.Add Anchor:=oApp.Selection.Range, Address:=hyperadd, TextToDisplay:=disp
oDoc.Close -1
oApp.Quit
Set oApp = Nothing
lbl_Exit:
Exit Sub
End Sub