The Chinese conversion - Simplified <-> Traditional feature (繁簡轉換) requires you to have the Chinese language pack installed. Please check the Office Language preference of your office suite.

Procedures: (Applicable for MS Word 2016/2019)

• Click "Start" -> "Programs" -> "Microsoft Office" -> ""Microsoft Office Language Preferences"
• Make sure you have the "Chinese (Tradition, Taiwan)" installed and set as default. If not, you can click the "Add additional editing language" and select "Chinese (Taiwan)". Then click "Add" to install it.
  
• Once the language pack installed, Launch MS Word and
• clik "Review" from the ribbon bar, then you can see the conversion button.