Installing SQL Server 2017, I get E: Unable to locate package mssql-server
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


Will tell apt to download a package list and look for the packages here,

https://packages.microsoft.com/ubuntu/17.10/prod/pool/


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 /pool/s)


https://packages.microsoft.com/ubuntu/16.04/mssql-server/pool/main/m/
https://packages.microsoft.com/ubuntu/16.04/prod/pool/main/m/


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.