Facebook denied CoSchedule's request to send this message. Facebook does not allow you to share Facebook event links with their API.
Why did I receive this error?
Facebook does not allow sharing of links to Facebook Events through their API (more info on that at "API and Other Platform Product Changes"). This affects CoSchedule as well as all other third-party applications.
What should I do to resolve this?
To resolve this issue you will have to share the event directly on Facebook. Or as an alternative, you could try using an image-type message and include a link to the event in the body of the message.
How do I prevent this error from happening in the future?
The only way to avoid this issue is to not share links to Facebook Events in messages you schedule through CoSchedule.