var endpoint1 = new RestAssured()

.Given()
//Optional, set the name of this suite
.Name("JsonIP Test Suite")
//Optional, set the header parameters.
//Defaults will be set to application/json if none is given
.Header("Content-Type", "application/json")
.Header("Accept-Encoding", "gzip,deflate")
.Host("jsonip.com")
.Uri("/endpoint1");

        endpoint1.When().Get().Then().TestBody("test 1", x => x.ip != null).Assert("test 1");

is not compiling and giving me error System.Exception : (application/json; charset=utf-8) not supported

Same for the below modified code from example:
var endpoint1 = new RestAssured()
.Given()
//Optional, set the name of this suite
.Name("JsonIP Test Suite")
//Optional, set the header parameters.
//Defaults will be set to application/json if none is given
.Header("Content-Type", "application/json")
.Header("Accept-Encoding", "gzip,deflate")
.Host("jsonip.com")
.Uri("/endpoint1");

        //Make a copy of the settings from above, but adjust the endpoint.
        var endpoint2 = endpoint1.Clone().Uri("/endpoint2");

        //Do a GET action with the first endpoint configuration
        endpoint2.When().Get().Then().TestBody("test 1", x => x.id != null).Assert("test 1");

        //Do a POST action with the second endpoint configuration
        endpoint2.When().Post().Then().TestBody("test 1", x => x.id != null).Assert("test 1");