According to the official Office documentation, I tried to use Yeoman's generator-office to generate WORD add-in, and then use npm start to start, but in the Taskpane it always showed as bellow:
Using the Microsoft Edge DevTools Preview tool, I can see the missing Office objects:
But when I visited the web page, I can see that the Office object is loaded successfully:
Why is the Office object unsuccessful when accessing this add-in in WORD?