I'm not sure where Steam puts the files, but you can find out:
Go into the game, right-click on the "General" tab in your chat window, and enable chat logging. In the chat window, it will show you where it's saving the log file. That's the same directory where the game is looking for the "Plugins" folder. Example:
Logging chat tab General to file C:\Users\username\Documents\The Lord of the Rings Online\General_20200607_1.txt
In Plugin Compendium, look under the "Configuration" tab and make sure the "Plugin Install Path" is the same. Example:
C:\Users\username\Documents\The Lord of the Rings Online\Plugins