You have a similar error in this thread
It turns out the oldest version available in the repos for focal is 13.2.0. So I reverted the gitlab to bionic:
deb https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu/ bionic main
deb-src https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu/ bionic main
To my surprise, the upgrade to 13.0.0 has just finished without problems. I can now go further to upgrade it to the latest. And switch it to focal when I get to 13.2.0.
In your case, since you are using debian 10 "buster", you might need to adjust the repo accordingly, as in here, knowing that Debian Buster is not be supported until GitLab 12.2.
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