It is required to add MS SQL database server in Plesk for Linux, but only MySQL and PostgreSQL are available under Plesk > Tools & Settings > Database Servers > Add Database Server.
Why cannot it be added?
This functionality to add MS SQL database server is yet to be implemented in Plesk.
Vote for such functionality on Plesk portal User Voice.
The top-ranked suggestions are likely to be included. in the next versions of Plesk.