Tally Prime can generate bills by sending invoice data to the portal through API credentials, so you don't re-key details. The setup uses an API user — not your web login.
Setup
Create an API user on the portal, then enter those credentials in Tally's E-Way Bill configuration. Full steps are on the generate in Tally page.
Generating from a voucher
Record the sales voucher with all mandatory details, then use Tally's E-Way Bill option to push the data and fetch the EBN into the voucher.
Fixing invalid credentials
The invalid login credentials error almost always means web credentials were used instead of API credentials.