You are missing the file abstractions/mysql. This happened to me when I tried to completely remove MySQL before reinstalling it. I fixed this issue by replacing the file with a copy from another server. The contents of my file looks something like this:
# ------------------------------------------------------------------
#
# Copyright (C) 2002-2006 Novell/SUSE
# Copyright (C) 2013 Christian Boltz
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of version 2 of the GNU General Public
# License published by the Free Software Foundation.
#
# ------------------------------------------------------------------
/var/lib/mysql{,d}/mysql{,d}.sock rw,
/{var/,}run/mysql{,d}/mysql{,d}.sock rw,
/usr/share/{mysql,mysql-community-server,mariadb}/charsets/ r,
/usr/share/{mysql,mysql-community-server,mariadb}/charsets/*.xml r,
Try creating the file and adding the above contents with:
sudo nano /etc/apparmor.d/abstractions/mysql