xUbuntu 18.04 の場合は、下記のコマンドを実行してください:
Keep in mind that the owner of the key may distribute updates, packages and repositories that your system will trust (more information).
echo 'deb http://download.opensuse.org/repositories/systemsmanagement:/Uyuni:/Master:/TEST:/1:CR:/Ubuntu1804-Uyuni-Client-Tools/xUbuntu_18.04/ /' | sudo tee /etc/apt/sources.list.d/systemsmanagement:Uyuni:Master:TEST:1:CR:Ubuntu1804-Uyuni-Client-Tools.list
curl -fsSL https://download.opensuse.org/repositories/systemsmanagement:Uyuni:Master:TEST:1:CR:Ubuntu1804-Uyuni-Client-Tools/xUbuntu_18.04/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/systemsmanagement_Uyuni_Master_TEST_1_CR_Ubuntu1804-Uyuni-Client-Tools.gpg > /dev/null
sudo apt update
sudo apt install debhelper