Unable to set up monitoring for root partition in Datadog

I am trying to set up a monitor in Datadog to track the disk space on an ubuntu EC2 instance and alert my team if it's running low. For some reason, when I attempt to set up the metric the root partition does not show up. I can see other partions like tmpfs and /dev/xvda15 but not /dev/root. Below is the query I am trying to use for my monitor.

((avg:system.disk.total{host:linux* ,!device:/dev/loop*,device: /dev/root} by {host,device} - avg:system.disk.used{host:linux* ,!device:/dev/loop* ,device:/dev/root} by {host,device}) / avg:system.disk.total{host:linux* ,!device:/dev/loop*,device: /dev/root} by {host,device}) * 100

I installed datadog with a user named "ubuntu". So I tried doing sudo -i to switch to root and install datadog there but the installation failed. I suspect this may have something to do with permissions.