To allow GST Zen to upload your Invoices and Returns, you must allow API access in your GST Portal.
Go to the Government portal, gst.gov.in and login.
After logging in, click on My Account in the top right corner of the page.
Then click on the Manage API access link.
Click on Yes, to enable API request. Set a time limit, preferably 30 days and Confirm.
You have now authorized GSTZen to upload invoices and file returns for you.