Not sure... I'll tag
@MrsFlynn in here to answer that question.
If none of those things have worked it may be that it is a game bug that needs a developers fix.
If you wanted to start a brand new game with the same sims... then you would need to save your sims to your library etc (I'm sure you know that) and place them into the new game.
The only thing I have found is a vague bug report but like you, they too couldn't find a way to solve it.
If you go into build mode and place the objects needed (dj booth, mixology bar, lights etc) does the party work as it should? If it does that may be the only work around that is feasible.
Another thing you could try is contacting crinrict by posting a message on the
USA official forums in their bug report section or by posting a bug report on the
EA The Sims 4 bug reports as I know she often asks for save files and then basically dissects them to problem solve.
If this is one of those bugs that needs a devs fix the quicker a bug report is made the quicker a dev may see it and look into it.
*edited. it just occurred to me
@DavinMcleod are you using any mods or cc?