Thanks for helping out. You are not observing the error because you have not yet enable razor intellisense. I can see this in your video as your tag helper is not purple. You can do this by going to Tools / "Extensions and Updates". Click on Online in the left column, and use the search box to locate "Razor Language Services." Install the extension and restart VS 2017.
After that your taghelper should be purple and you should see the error when you re add the taghelper. Please note: Sometimes the error does not show until you retype taghelper.
Me providing you with a sample project will not help unless you have the intellisense working and if you do this the error should show up in your project, I created a new project just like yours and the error showed up.