EJ W. There is a bug where we are returning the incorrect error message when the cover_image_file_id is invalid. We'll have a fix out shortly! You can remove the cover_image_file_id from the payload, or use the File Upload API to use a real file.
That said, there are several other IDs in the example data that serve as samples and don't point to real records, such as the Slack channel IDs and user IDs. You'll need to modify them for your real Slack workspace data accordingly.