Hey Swagato,
It appears you might just have just needed to wait for the API to finish deploying, or for the API Explorer page to load fully. I can see the pseudoGen API v1 has successfully appeared. You can check the Cloud Console's Logs Viewer for the relevant service/version of the app you deployed to in future, as well, to see if there were any complications with setting up the API.
Regards,
Nick
Cloud Platform Community Support