You can use wix built-in function called CloseApplication, it was designed to do that exactly thing: https://docs.firegiant.com/wix/schema/util/closeapplication/

<util:CloseApplication 
  Id="CloseApp" 
  PromptToContinue="yes" 
  Description="The setup wants to close Word."
  CloseMessage="yes" 
  Target="Word.exe" 
  RebootPrompt="no" 
/>

BTW, I really don't think you need to close Word (like I've stated in the comment), if you get an error during installation it may be something else.

You can use wix built-in function called CloseApplication, it was designed to address that exactly scenario: https://docs.firegiant.com/wix/schema/util/closeapplication/

<util:CloseApplication 
  Id="CloseApp" 
  PromptToContinue="yes" 
  Description="The setup wants to close Word."
  CloseMessage="yes" 
  Target="Word.exe" 
  RebootPrompt="no" 
/>

BTW, I really don't think you need to close Word (like I've stated in the comment), if you get an error during installation it may be something else.