I also had the issue where the option to 'take a piece' was grayed out and the message given was that I must wait for the newlyweds to take the first slice. I tried with both the bride and groom and neither could take the first slice. I moved the wedding cake to different surfaces and nothing helped.
The wedding was held in Myshuno Meadows. I had brought the cake with me, and when it didn't work, I had one of my Sims bake another cake at the venue and it didn't work either. There were other options available (including 'remove wedding topper').
As an FYI, I opted to hire both a mixologist and a caterer and for the wedding and neither showed up.
I then quit the game without saving and when I reopened I tried having the wedding at their home (large penthouse in Fashion District of San Myshuno) and the option to 'take a piece' was still grayed out for both the bride and groom. Because I quit without saving, I had to have the cake made again, so the same cake was not used for my second attempt.
It has been a long time since I've held a wedding, so I'm not sure when this problem started.
I play on Windows 10, 64 bit.
Game was repaired after my first wedding attempt failed.
Game is fully updated and I don't use mods or cc. I have all packs but the Vampire pack.
FYI, I have a copy of the game where the wedding event is still on, with the only task remaining is to 'take first slice'.
Thanks and regards,
Jennifer