If I remember right (and I’m not positive I do) you have about 30 days to leave a review under current policies. You could try asking the seller to redeliver. That can re-trigger the review option but I’m not sure how long you can do that.
It is usually not that difficult to leave a review, but Fiverr had to restrict the time because of review system abuse. There were unfortunately buyers who would come back even months after a purchase, attempt to manipulate sellers into low priced or free work and if they refused, a bad review was left. I doubt it was a high percentage since most buyers on Fiverr and great, but there are always some bad apples. You could also try contacting Customer Support to see if they can fix the issue if you think there might be a glitch on that particular gig review.