SoftEther is an Open Source VPN Server, an alternative to OpenVPN. It's thought to be the world's most powerful and easy-to-user multi-protocol VPN software. Our article concerns how to setup SoftEther on Ubuntu Xenial Xerus Linux.
have you tried using ppa like https://launchpad .net/~paskal-07/+archive/ubuntu/softethervpn ?
I suppose updating software with deb-packages is easier than downloading new binaries from site.