Apparmor was (is) not loaded by the kernel. I made a report : https://github.com/raspberrypi/linux/issues/3196
This one (3196) was closed because an old thread (1698) was open about that problem.
That thread (1698) has been relaunched, the work has been done and tested, then proposed for merging.
You can say there that your are also waiting for this : https://github.com/raspberrypi/linux/pull/1698
In the meantime, you have to recompile the kernel if you want to use apparmor. Read mainly this message
Pi 4 with 4Gb memory, DVB TV pHAT ------------------- Desktop : Core i5 Linux MINT 19.3