What are the requirements to use App Lock feature on Linux?
On Linux, you need to have libsecret. So they must be installed before you can enable App Lock.
To install these libraries:
On Debian-based system (e.g. Ubuntu):
sudo apt-get install libsecret-1-dev
On Arch Linux:
sudo pacman -S libsecret
The process is mostly identical for other systems/package managers. Certain systems have these packages preinstalled so you don't need to anything.