After installing Debian Testing "Bullseye" mid March 2020 I got an error trying to run apt update.

E: The repository 'http://security.debian.org./debian-security 
bullseye/updates Release' does not have a Release file.

Unfortunately, the Debian maintainers managed to let a bug creep into the /etc/apt/sources.list. It’s called "Testing" for a reason, I guess.

The offending lines are this.

deb http://security.debian.org/debian-security/ bullseye/updates main
deb-src http://security.debian.org/debian-security/ bullseye/updates main

Note "bullseye/updates", which is where the error is. Change those two lines to look like this.

deb http://security.debian.org/debian-security/ bullseye-security main
deb-src http://security.debian.org/debian-security/ bullseye-security main

After that, the update will work. I have noticed that in a later version of the installer this bug has been fixed.