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This looks like a problem with name resolution on your machine. It's saying it can't find the name "git.openstack.org" in DNS, but it's resolving for me with no problem. Do you have trouble cloning Git repositories from anywhere else? You could try git clone https://opendev.org/openstack/devstack instead, for example (the URL you've been trying is an old redirect maintained for compatibility with the previous place that repository was hosted).