You installed the extension, but you must configure it or else it will not operate at all.

It should take you about 30 seconds.

Here's the 3 quick steps. At the bottom of this page, there's also a very short video you can watch, if you like.

1. In Chrome, Goto Menu (icon looks like ⋮ or ☰) > More Tools > Extensions

   

2. Find this extension (the name is "Hide Incognito Mode") in the list and click the "Details" button

   

3. Enable "Allow in incognito"

   
4. You're done

Here's a video showing the steps: