An order has been canceled today and now my order completion rate is 80% . How can I overcome this?
You will overcome it by completing more orders. Sorry about the cancelled order.
Keep in touch with the customer and ask him/her nicely for the reason(s) and if there’s anything you can do help them out.
Just be polite and show them that you care as much for the success of their project as they do like a colleague.
If they don’t respond or don’t give you any helpful information, just wish them best of luck and let them know that they can count on you for future projects.
Until then, as @barrisam said, you have to move on.
DO NOT PESTER THE BUYER FOR REASONS.
Fiverr may flag you for spamming.
You don’t want that.