I do not think that “all” is a valid setting for the discourse post event allowed on groups site setting. Try setting it to trust_level_0 or trust_level_1. Either of those values should allow all members with that trust level or greater to create events.
On my site, I am not seeing “all” as an option for the discourse post event allowed on groups site setting, but I am seeing “everyone” an an option. I think this is a bug. The “everyone” group is a special group that includes all users. It also includes anonymous users for sites that do not require users to login to view the site. The “everyone” group can generally not be used for settings like the discourse post event allowed on groups setting. I’m going to create a ux topic to suggest that the “everyone” group is not included in that setting’s drop down menu.
Edit: Remove the "everyone" group from event allowed groups setting
Where are you seeing this parameter? A screenshot would be useful if you can provide one.