The Agent Compatibility Matrix in the CI Server Settings section contains a table with all Agents as column names and all Configuration Stages as rows. The intersecting table cell contains the compatibility result for that Agent and Stage. The following image shows a typical compatibility matrix.
The 
imagine means the Agent can run that stage.
The 
image means the Agent cannot run that stage.
The reason an Agent is incompatible with a stage is displayed when hovering over the . In the following imagine, I've hovered over the incompatible imagine for Build stage in the todotxtnet project's Release configuration on Agent continuagent.
The tooltip that pops up shows a list of properties the stage requires but the Agent doesn't have. In this example, the stage requires access to the MSHTML and Git executable. Since those executables don't exist on continuagent the stage cannot be selected to run on that agent.