Sorry to hear about your situation. We have all been there, trying to reason with an angry and unreasonable seller. The problem is that Fiverr has created a perfect environment for bullies (unfortunately Fiverr are themselves the worst bullies). What I mean by this is that us sellers have ZERO rights. The buyers can treat us anyway they want. Even if you bend backwards to comply with everything they want they can still treat you like an animal and get away with it. They know that the rating system is working on their behalf. They will always have that threat over you. I would liken to work as a seller on Fiverr as constantly working under the threat of a gun to your head. By a seller having a bad day, or by Fiverr staff themselves deciding to punish you. It does not matter if you deliver a high quality service or product, if the seller decides they want to take their anger out at you, they will be allowed, even encouraged.
So in the end, if even if you cancel the order, the result will be in worst case that you worked for free, and even worse, you will be left a review, since its possible to leave a review for canceled orders (only possible for the buyer, not by the seller, go figure), leaving you with less or maybe no customers at all in the end. All you worked for, gone. At least Fiverr made some dollars.