I asked the same question to Fiverr support and here is the reply:
"While the buyer is not obligated to leave feedback, you can kindly remind them about it."
"It's important to note that asking for positive reviews, and ratings, or requesting another user to edit or remove their review is against our guidelines."
And here is my opinion:
You can ask for a review to a customer on Fiverr. However, there are a few things to keep in mind:
You cannot ask for a positive review specifically. This is against Fiverr's Terms of Service.
You should only ask for a review after the order is complete and the customer has had a chance to review your work.
You should ask politely and professionally.
Here are some examples of how to ask for a review on Fiverr:
"Hi, I hope you're happy with the work I did on your project. I would really appreciate it if you could leave me a review. Just click on the 'Leave a Review' button on the order page."
"Hi, thank you for working with me on your project. I'd love to hear your feedback on my work. Could you please leave me a review on the order page?"
"Hi, I hope you're satisfied with the final product. Could you please take a moment to leave me a review? It would really help me out."