Unable to locate package monodevelop

When installing Altman, monodevelop is required, and the installation prompts that it cannot be located. The following is the solution:

1. Add the package to the repository

sudo apt install apt-transport-https dirmngr gnupg ca-certificates
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb https://download.mono-project.com/repo/debian preview-stretch main" | sudo tee /etc/apt/sources.list.d/mono-official-preview.list
sudo apt update

2. Install mono

sudo apt-get install mono-devel mono-complete monodevelop