When the setup is going to install gnocchi-api and libapache2-mod-wsgi-py3, It tries to remove cinder-api keystone libapache2-mod-wsgi nova-placement-api openstack-dashboard packages from my openstack queens installation, I don't know what I have to do. Thanks for reading guys.
I faced a similar issue. Struggled with it a lot for days. Are you trying via pip install? Anyway, I reinstalled keystone, openstack dashboard to get the system back to it original state. Once that is done, try installing gnocchi via apt install.
If you see gnocchi logs as it installs gnocchi, you will observe that it for some reason removes openstack dashboard, keystone and placement api, also. So, no service was running. That's when I reinstalled dashboard, keystone and placement api by following steps mentioned in stamdard openstack queens guide.