Hi @hiark,
Could you share a screenshot of your Flow configuration?
It seems that you don't have permission to change this calendar. This calendar is set to "Read-only", so you can't make any changes to it.
First please make sure that the Calender id you choose is the default "Calendar" or your custom calendar, not the "United States holidays/Birthdays" calendar.

Or you could check Share and see if "People inside your organization" is set to someone else who can't edit the corresponding calendar.

Please take a try again.
Best Regards,
Barry