When checking the challenge  I get a persisting issue with the "Phone Plan" external object 

Displayed message is : 
Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.ExternalObjectException: data.api.DataSourceException: This data is no longer available. The "Phone_Plan" table in the external data source is currently unavailable. Try again later or contact your administrator for help.


I have tried with a new developer ORG and I get the same error.

Thanks. 

• Create an OData 2.0 External Data Source named 'Mobile Devices' using the following URL: https://phone-odata-demo.herokuapp.com/devices.svc/
• Use the 'AtomPub' format and the 'Anonymous' Identity Type for the 'Mobile Devices' External Data Source.
• The 'Mobile Devices' data source must validate and sync the 'phone_plans' table to result in an external object with the API name 'Phone_Plan__x'.
• The 'Mobile Devices' data source must validate and sync the 'phones' table to result in an external object with the API name 'Phone__x'.
• Note that you will need to adjust the Object Names for the two external objects from the default setting after the initial sync.

all the thing have been done but i am getting following errorwhile checking challenge.