The applications Radarr, Jackett, and Sonarr all depend on mono. Perform the commands below to remove as needed.
Also, updates were pushed to make use of this on the installer packages. So make sure you’re updating your QuickBox. 
Additionally, you may need to make use of the stapled version of mono. So do the following:
rm -rf /etc/apt/sources.list.d/mono-xamarin.list
apt -y remove mono-devel mono-complete libmono-cil-dev
apt -y autoremove && apt -y autoclean
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/ubuntu xenial/snapshots/5.8.0.127 main" | sudo tee /etc/apt/sources.list.d/mono-snapshot-5.8.0.127.list
apt -y update && apt -y upgrade
apt -y install mono-devel libmono-cil-dev ca-certificates-mono