Part of the answer is in the help files here: http://support.fiverr.com/hc/en-us/articles/205167778-What-URLs-can-I-use-in-my-profile-or-Gig-description-
So, if you read that you’ll see that YouTube is allowed as long as your link doesn’t lead to a channel that does violate the ToS. For that, you need to read the terms on your own because there are several parts that apply and it would take a while to locate and paste every part in the forum for you.
Just read the terms so you don’t get your account banned. For example, if the URL you put in your gig has ways to contact you or give you money outside Fiverr, those are violations. It’s better to know the ToS sooner than later since a gig taken down by Fiverr usually goes down permanently.