sqlcmd not available after installing mssql-tools

Quoting the page you linked to:

Optional Step: Create symlinks to 'SQLCMD' and 'BCP' under /usr/bin/.

ln -sfn /opt/mssql-tools/bin/sqlcmd-13.0.1.0 /usr/bin/sqlcmd 
ln -sfn /opt/mssql-tools/bin/bcp-13.0.1.0 /usr/bin/bcp

1. Add the path either to ~/.bash_profile or ~/.profile if you don't have .bash_profile in your home directory.

   export PATH="$PATH:/opt/mssql-tools/bin" 
   

2. Add the following path to your ~/.bashrc file

   export PATH="$PATH:/opt/mssql-tools/bin"
   

3. Don't forget to update the path configuration by issuing the following command in your terminal window:

   source ~/.bashrc