Looks like your integration response key mappings are missing in the Integration Response tab. Take a look at this document - https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-websocket-api-integration-responses.html particularly the below section. Either a $default mapping has to be present or a /2\d\d/ response key has to be present for the backend return code of 200 to be mapped back in the response.
**For non-proxy integrations, you must set up at least one integration response:
Ideally, one of your integration responses should act as a catch-all when no explicit choice can be made. This default case is represented by setting an integration response key of $default.
In all other cases, the integration response key functions as a regular expression. It should follow a format of "/expression/".
For non-proxy HTTP integrations:
API Gateway will attempt to match the HTTP status code of the backend response. The integration response key will function as a regular expression in this case. If a match cannot be found, then $default is chosen as the integration response.
The template selection expression, as described above, functions identically. For example:
/2\d\d/: Receive and transform successful responses
/4\d\d/: Receive and transform bad request errors
$default: Receive and transform all unexpected responses
For current limitations of template selection expressions, see Template selection expressions.**
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