This was solved through mail. Reposting for everyone.
In your test case GST number is wrong. So you got that message. However to sum up here are the response codes you will get.
401 - You are not authorized to access data. (IF API key is wrong)
402 - Trial is over, You need to recharge your account.(To recharge account)
400 - If GST number is wrong or GST server is down. In 99.99% cases GST number will be wrong. We are working to keep both these separate.
402 - Subscription period is over.
In case API does not response it will give code 400 with message "Bad Request".