discourse post event allowed on groups setting drop down gives the “everyone” group as one of its options. When selected, the expectation is that everyone will be allowed to create events, but this is not the case. To allow any user on the forum to create an event, the trust_level_0 group would need to be selected.
I think that similar issues with the “everyone” group being included in settings dropdown menues has come up in the past. Probably the setting is just pulling in all groups, but the “everyone” group isn’t a real group and should be excluded from most of these settings.