I had this issue with a number of party types. I tried a house party, a dinner party and a keg party. Then I looked online to see if other people had this issue. The slob trait glitch kept coming up, so I tried modifying my Sim and removing his slob trait. Next party I threw, I had no issue and achieved the milestone.
Maxis forgot to add the exclusive hidden buffs that Slob Sims get when in a Dirty and Filthy floor in the objective tuning of the aspiration. There are other objectives Slob sims are unable to complete for this same reason, like cooking meals on a Dirty or Filthy floor and inviting guests in a Dirty or Filthy house. I fixed that along with other things here for anyone interested: Bust the Dust Rebuffed.😇
