Hi, @Aimthedame, if you have access to the calendar and are a member of the site / team that the calendar belongs to, then you should have the correct rights.
Either way, that would be a question for a SharePoint community or a Microsoft Graph / 365 community, I believe.
If you are still prodding, then there is a chance that you actually do have the authority to edit / add to the calendar. Just format your POST calls a little differently.
Addressing the correct Accept/Content-type headers, with the right ODATA parameters within them, is often an easy fix.
Plus, if you just use no headers, and the SharePoint Send an HTTP request to SharePoint action you should be OK with minimal data entirely! 🙂
Otherwise, you'll want to use the Groups or Outlook connectors along with their HTTP actions and the requisite Graph API calls.
If there are still permissions issues after trying a ton of different header combos (including none) then I hope that you get an answer here ... but you may also wish to hedge your bets with a more appropriate community, as previously mentioned. 🙂