Without the sender unit label, no e-invoice goes out
There is a field in the Sovos setup that is easy to miss: the sender unit label. It is mandatory for sending e-invoices and is defined for you by Sovos. Leave it empty and authentication passes, the connection looks established — but the invoice sending step is rejected. So the setup looks successful while the actual job cannot be done.
The identity side consists of a web service username and password, issued by Sovos rather than generated in a panel. Your tax number is also used as the sender identity in sending, status queries and downloads.