There is no built in manual, only a link to online help. Maybe that's missing in your Linux version. Maybe also you have an older version. The present official Linux release is 2.1.2 or 2.1.3.
This link is for 2.2.0 rc, but mostly valid for 2.1.3 also: https://librecad.readthedocs.io/en/latest/index.html