Following when I clicked check challenge and there seems to be end point is down

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.CalloutException: IO Exception: External server did not return any content

Following when I clicked check challenge and there seems to be end point is down

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.CalloutException: IO Exception: External server did not return any content