The AACs file is a list of Disk Identifiers and Decryption Keys needed to play Blurays. Normally you buy a piece of software that will have its own list, but because these are all proprietary you can use community constructed AACs lists instead. The Arch Linux wiki has an extensive page covering different decryption methods.
You can play Blurays in VLC through the following steps
Install the required libraries
sudo apt install libaacs-dev libbluray2
Download the KEYDB.cfg file. More info on this site.
Extract the .ZIP file
Inside the keydb_eng folder is
keydb.cfg rename this to
KEYDB.cfg (It's case sensitive.) and move it into
~/.config/aacs (You might need to create this folder manually if it doesn't exist.)
Open VLC and try to open the disk. If the decryption key is known and present in the database then it should work fine, otherwise if it's a very recent Bluray you can download a new copy of the database at a later date and hope someone has added the decryption key for your disk.
If you're using the Snap version of VLC:
If using the Edge candidate of the VLC snap this will work fine, otherwise there's a bug where it looks in the wrong place for the AACs file right now. Simply copy the
~/snap/vlc/x1/.config/aacs/ and it will work.
cp ~/.config/aacs/KEYDB.cfg ~/snap/vlc/x1/.config/aacs/
If you implemented this workaround and you're now switching to a version of VLC where the bug is fixed, then you should remove
~/snap/vlc/x1/.config/aacs/KEYDB.cfg and ensure the KEYDB.cfg file is inside