You should only ask a client email if it’s required in a website page or anything else.
And if you are going to get the client’s email for direct contact, that will lead to ban. And remember be honest with fiverr give a 20% to them because they deserve it and far away then deserve they own it.
And if you want to ask then place a note saying " This email is for order fulfill and attach a screenshot of the website page or flyer or poster anywhere necessary
Also, I recommend checking Fiverr Seller FAQ’s.