MicroStack tries to be helpful, and let you skip the step of sourcing the openrc, by providing its own .rc file in /var/snap/microstack/common/etc/microstack.rc. The wrapper around the microstack.openstack command will source this rc automatically for you.
Does the command work if you start in a clean terminal, without sourcing the openrc that you downloaded?
If so, you might want to file a bug against MicroStack – the openrc file you get from the dashboard might be missing a bit of info necessary for making the command work.
You can probably fix the microstack command line tools by running:
sudo snap set microstack config.credentials.keystone-password=<your password>
That will prompt microstack to overwrite the default password in its .rc with the new password.
You can then do a comparison between /var/snap/microstack/common/etc/microstack.rc and the rc you downloaded, and file any missing pieces in the downloaded rc as a bug against MicroStack.