i got the solution for the same that to edit Phone Plan from External objects but before this error I was getting another error that was:T
There was an unexpected error in your org which is preventing this assessment check from completing: System.ExternalObjectException: An error occurred while connecting to the external system. Please try again, or contact your administrator. Attempted to reach this URL: https://phone-odata-demo.herokuapp.com/devices.svc/phone_plans?$top=1&$inlinecount=allpages&$select=Id. Error received from the external system: 503: Application Error..... after solving this by not editing "Phone Plan " now i m getting the above error message.
There was an unexpected error in your org which is preventing this assessment check from completing: System.ExternalObjectException: An error occurred while connecting to the external system. Please try again, or contact your administrator. Attempted to reach this URL: https://phone-odata-demo.herokuapp.com/devices.svc/phone_plans?$top=1&$inlinecount=allpages&$select=Id. Error received from the external system: 503: Application Error.....
after solving this by not editing "Phone Plan " now i m getting the above error message.