Description
The user running the Find Routes tool available with the ArcGIS Online route service does not have privileges to perform optimized routing; that is, they do not have privileges to use the Rorder Stops to Find Optimal Routes parameter.
Solution
If you are using an API key to execute the ArcGIS Online route service with asynchronous execution, ensure that the API key has been granted access to the optimized routing location service or uncheck the Reorder Stops to Find Optimal Routes parameter (set it to False).