Download from GitHub, supports Linux Mint 20 - https://github.com/OpenBoard-org/OpenBoard/releases
OpenBoard per se is a great tool for teaching and creating educational content. But the advertisement says "Easy to install". That's not true. The installation process is a pain in the back. It doesn't install in recent versions of Ubuntu and derivatives (Linux Mint 19.3, KDE Neon, etc.). Best you can do is to install it from the flat pack system. Open board's original size is about 50 MB, and maybe 100 MB when installed, but the installation through Flatpack takes as many as 2.7 GB after installation! If you can live with that, then use it! You won't regret.