Normally, a distro packages things in a specific format, so for instance adding a repository like
deb [arch=amd64] https://packages.microsoft.com/ubuntu/17.10/prod zesty main
apt to download a package list and look for the packages here,
However, with Microsoft that's not the case. They have a repository for 17.10 (as they should) which has all their packages for Ubuntu, but they have another repository specifically for SQL Server. To make matters more confusing, that SQL Server repository is in the tree of 16.04.
So they have both (pay attention to the
You need the
16.04/mssql-server repository. You can add that with
sudo add-apt-repository "$(curl https://packages.microsoft.com/config/ubuntu/16.04/mssql-server-2017.list)"
You may think because you have the
17.10/prod and you have SQL Server installed that you don't need the
16.04/prod repository. That's incorrect, you'll need that too if you want the SQL Client
sqlcmd provided by mssql-tools
sudo add-apt-repository "$(curl https://packages.microsoft.com/config/ubuntu/16.04/prod.list)"
I did file a doc bug upstream with a description of the confusion so maybe they'll fix it someday
If you find your system in an inconsistent state for any reason, you can always remove SQL Server and start over.