@ReeniRoo Possibly a bug then, unfortunately the Host Party tutorial should trigger for all accounts upon reaching Level 6. After a week then, you can "Party On" (host parties).
Or maybe, your Sims have been in events everytime and that is why you can't host a party.
Let them finish their events first. For example, as you see in the picture at the top, the Actively Selected Sim is in an event (having pink border and a star icon), let them finish it first, or have another free Sim (has white border) host the Party instead.
Remember, Parties can't be started when your Sim is an event, after starting the Party, however, your Sims can go to their Jobs, Hobbies, and Relationship Events.