Lidarr's AppData folder contains the the databases for logs and general app information (artists, albums, tracks, history, and most settings), log files and the config file that contains settings required to start the web server. This path is listed on the System Status page in Lidarr, but if you are unable to access it, here are some common locations for it:
Windows
C:\ProgramData\Lidarr
Ubuntu
/home/<user>/.config/Lidarr (~/.config/Lidarr)
OS X
/Users/<user>/.config/Lidarr (~/.config/Lidarr)
Synology
/usr/local/lidarr/var/.config/Lidarr
QNAP
/share/MD0_DATA/homes/admin/.config/Lidarr
/share/CACHEDEV1_DATA/LIDARR_CONFIG