"message": "Bad Request: Configuration must be a valid YAML document"

Without the YAML file you're using it's not possible to say where the problem is.

A common problem is if you've copied the text on the previous page into (say) Microsoft Word and then saved it. Some editors (Word is the main offender here but there are others) will change the formatting, insert "smart quotes" and that will make the YAML file non-compliant. Always use a plain text editor like Notepad for copy/paste of code.