The issue with your script is that you are trying to build here an express app (not supported) and that the body of exports.onExecutePostUserRegistration function is empty so the Action does nothing. The logic within the body can be build on the flow-specific methods and properties listed in the docs.
When attempting to POST something to an external service or as part of the flow, don’t forget to add relevant secretes.
In terms of NPM modules management and limitations I found this doc useful -
I don’t think the issue is with modules installation. In the link I’ve sent you the last time, there are errors codes you would receive for the missing dependencies.
You can not create a webhook server (or any server) on Auth0 infrastructure as part of Auth0’s actions.
You do can send registration transaction data to an external server by providing the full address of the server as well as including authorization data in the request if that is required by your webhook server.