It’s so weird. Before I was able to tip and now I can’t. I’ve tried varying amounts under the custom tab.
You may want to open a ticket with Customer Support: https://www.fiverr.com/support_tickets/new
It’s taking longer for them to reply to ticket, some people have reported up to 10 days.
If, in the meantime, you’d still like to tip your seller, you could ask them to send you a custom order for the value you want to tip them. Although more cumbersome, it will work just as tipping would in term of amount paid/received.