This happened to me the first time I installed:
volumio-2.861-2020-12-22-pi and
volumio-2.873-2021-02-19-pi
If I try to add my source during the first config, it fails. However, once I complete the config I can try again and it works. This may be due to the fact that I get a prompt the second time for my username and password for that shared volume. That usr/pwd popup doesn’t appear if you try to connect during the first config.