Problem
When trying to access a target computer, you get the error "Could not connect to agent or could not get device list".
Resolution
Select the information you want to collect from the target, and ALSO select "Install or Update Agent Modules" > "Modules Only" to push the modules to the target.
Cause
SCCM Deployment can install the agent, but the agent modules still need to be pushed/added through the program UI itself.