Setup repository and install dependencies

mkdir ~/src
sudo apt install gnupg ca-certificates
sudo apt autoremove
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
sudo apt update
sudo apt install mono-devel

Download and install Radarr

cd ~/src
curl -L -O $( curl -s https://api.github.com/repos/Radarr/Radarr/releases | grep linux.tar.gz | grep browser_download_url | head -1 | cut -d \" -f 4 )
ls
tar -xvzf Radarr.develop.*.linux.tar.gz
sudo mv Radarr /opt/

Setup Radarr to launch with SysetemD

sudo -i
cat <<EOF > /etc/systemd/system/radarr.service
[Unit]
Description=Radarr Daemon
After=syslog.target network.target

[Service]
# Change the user and group variables here.
User=tube
Group=tube

Type=simple

# Change the path to Radarr or mono here if it is in a different location for you.
ExecStart=/usr/bin/mono --debug /opt/Radarr/Radarr.exe -nobrowser
TimeoutStopSec=20
KillMode=process
Restart=on-failure

# These lines optionally isolate (sandbox) Radarr from the rest of the system.
# Make sure to add any paths it might use to the list below (space-separated).
#ReadWritePaths=/opt/Radarr /path/to/movies/folder
#ProtectSystem=strict
#PrivateDevices=true
#ProtectHome=true

[Install]
WantedBy=multi-user.target
EOF
exit

Enable, start and verify the Radarr service…

sudo systemctl enable radarr.service
sudo systemctl start radarr.service
service radarr status
netstat -an | grep LISTEN | grep 7878