This error means one of the two things: 1) you don't have a system-wide installation of python; 2) your have python installed but it is not in its standard directory (/usr/bin/python). To figure this out, type:
This will tell you where your python is, or it will again say No such file or directory. If the latter happens, you need to install python. If the previous command gives you a different python location, say /bin/python, you need to copy that line and paste it as a first line in hisat2-build. First find where hisat2-build is:
Then open this file and instead of the first line being #!/usr/bin/env python, enter #!/bin/python.