Well that’s unfortunate since I don’t have Lovestruck active and still get this as a bucket list item.
Edit:
I actually got this to work. The date it worked on was admittedly in a modded save, but I don’t have any dating/relationship mods so hopefully this will work for other people as well - you don’t declare a date the best date ever while on it. You declare it the best date ever afterwards it finishes - and not even immediately after.
Specifically my sims went on a date, got flirty, got engaged, and got gold on the date. Then when the date ended, I took them on vacation with their daughter to get married. I finally had the “declare best date ever” option when I clicked on my sim’s new spouse after the wedding.
I would assume a wedding is not required? But perhaps completing the date, then continuing to spend time with your date afterward, possibly while still in a flirty mood.
