Check whether the entered backend service address is correct and make sure that the backend service can be accessed normally.
If your backend service is on ECS, check the security group settings to see if it can be accessed externally. Make sure that the security group allows access to the egress address of the API gateway.
- Get the Egress Address
View the instance to which group belongs from Apigateway Console->Publish APIs->API Groups->group details
Get the Egress Address from the instance
- If you are using an intranet IP, make sure your backend services are in the same Region as your APIs.