Welcome! I use Reaper on Ubuntu.
When asking questions about troubleshooting, it's really helpful to say:
- What flavor of Linux: (Manjaro, Ubuntu, etc.)
- What *version*
- Hardware: CPU/RAM/Video Card
- Version of Reaper
All of that info makes it easier for people to help you.
There is a forum specifically for Linux (near the bottom of the main forums page).
Also, in that Reaper for Linux forum is a thread about ReaLearn:
https://forum.cockos.com/showthread.php?t=266954
which might be what you are looking for.
Edit: make sure you read all of the posts in that thread.
As to the sidechain compression issue, that is definitely *not* Linux related. I do sidechain compression all the time.
Lets continue this discussion in the Linux forum.
See you there!