openstack - E: The repository 'http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu disco Release' does not have a Release file

I have been trying to install Devstack and this happened while "./stack.sh".

You should remove this PPA from the system with

sudo add-apt-repository -r ppa:jonathonf/python-3.6

and then run

sudo apt update

to get actual package indexes.

Please note if you really need the packages from this PPA, then consider to contact JonathonF and ask him about packaging for disco. See example text in this answer.