Answer
Add ?odata-debug=json to the end of the request URL — the resulting OData debug information in the response is the way to get past the generic 'Metadata not found' message and see what's actually going wrong underneath it, since this error can have several different root causes. In one confirmed case, the actual cause turned out to be an active Workflow tied to the Customer Order area interfering with the create call; disabling the relevant workflow event resolved it. Since 'Metadata not found' is a fairly generic symptom, treat it as a starting point for using odata-debug output to find the specific underlying cause (which can range from workflow interference to a missing/misconfigured projection) rather than assuming any one fix applies universally.