@Byron Hayes Thanks for reaching out.
The endpoint becomes available, when you enable Lost mode on a supervised IOS device from managed devices list on Portal. The device must receive the commands.
Once the details are uploaded by the device, the same can be found under devicemanagement/manageddevices
It requires that the supervised iOS device be placed into lost mode and the Locate device command to be issued in the portal and received by the device. Once that's done you query the deviceManagement resource and the location appears as an additional attribute on the device.
If any further queries around this, do share the following details to azcommunity[at]microsoft[dot]com
tenant id/tenant name, subscription id.