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