Learn English – Is it “if you need further instruction” or “if you need further instructions”

Yes, use the comma. It makes the sentence clearer than if there's no comma. Sometimes commas are unnecessary, and sometimes they help. In this case, I had to read the comma-less sentence twice to separate item and click. Readers don't like have to read the same thing twice to be able to understand it. If what you're saying is inherently difficult to understand, that's one thing, and the readers have to deal with their own intellectual limits. But if it's merely a matter of writing mechanics and punctuation, it's your job as a technical writer to make everything you write as clear and as easy to understand as possible. That's what I try to do all day long.

He is mistaken. A correct formulation would be (notice the that):

That is exactly why I didn't read your text.

Why I didn't read cannot stand by itself because it lacks an object (that and text in the above example).