You should've had a birthday party in the tutorial. It lasts 24 hours and then you can't have a party for a week.
I do not work for EA
This is the exact issue I'm having. I'm almost level 25 and have not been able to host a single party. The button is greyed out most of the time but sometimes is green, when it is green I press on it and it says "Requirements not met". I have three Sims now and don't know what I'm doing wrong.
April 2018 - last edited April 2018
@01Reinier & @JuliaLouise1 thank you so much for your replies. I did think that initially when I had the one sim but I completed the current event and still was unable to host a party. I then thought maybe it was because I'd chosen both traits already so waited until I created a second sim before trying again. This sim was available with no active events and had recently chosen the first trait, I was still unable to host a party. I now have three sims, have played four one now retired and have still been unsuccessful.
I'm thinking it must be a bug because I didn't have an option to host a party in the tutorial. I remember parties being mentioned and needing decorations but I never had the option to host a party.
I feel it is a major setback as I'm playing my sims, I like to have the full experience.