How to start LaunchPad service of SQL Server on Ubuntu?
Linux Support Roadmap for SQL Server Machine Learning Services:

"Machine learning using R or Python in-database is not currently supported in SQL Server on Linux. Look for announcements in a later release.

However, on Linux you can perform native scoring using the T-SQL PREDICT function. Native scoring lets you score from a pretrained model very fast, without calling or even requiring an R runtime. This means you can use SQL Server on Linux to generate predictions very fast, to serve client applications."

The stand-alone Machine Learning Server is already supported on Linux.