The issue was my initial attempt at configuring the list file.
Here is the correct configuration:
deb [signed-by=/usr/share/keyrings/baculum.gpg] http://www.bacula.org/downloads/baculum/stable-11/debian bullseye main
deb-src [signed-by=/usr/share/keyrings/baculum.gpg] http://www.bacula.org/downloads/baculum/stable-11/debian bullseye main