It told you to print a diamond with a certain absolute width. Something cannot be (-x) units wide in absolute terms. Therefore you cannot accept the request for a negative width. You might as well ask why it doesn't specify not to accept a dictionary of the word "yo" repeated. It's simply outside of the logical validity of the task.
Sorry for being crude, but expecting to be explicitly told not to do things that are clearly inappliccable is just picky. You won't be led by the hand in real programming. In fact I think these tasks are a bit stupid at times, with how they remind you to not accept empty strings for algorithms that can only apply to ones that are not. If you do programming, knowing what kind of variable to accept is the very basics.

That being said, I'm no expert. People have different backgrounds and contexts for reacting to different things. But not being told every minor detail is just something I think everyone should get used to. The entire point of programming is to invent.

well, I'd like to see you print a negative asterisk. Or eat a negative apple.

I think it's self-evident that you don't accept negative input. (or input below 2, for that matter)

I'm afraid your reading of the description was a bit too fast: 101-107 is not a 6-gap because there is 103 in between. This problem was already posted and you can see it below.

I see your solution is in Java -- messages indicating what failed are now added! Typo also fixed!