“Here is why” means “here is why the word knowledge has not been used correctly in your post”.

Knowledge is the right word to use in this context, but the way you have used it is not correct.

And if you click on “here is why”, which is a link, you’ll be taken to the dictionary page for “knowledge” with grammar information and lots of examples which show you why the word knowledge was not used correctly in your sentence and how you can correct it.

The idea, as you have mentioned yourself, if for you to work things out for yourselves, rather than me providing answers. :-)