What happens when you try to send a task to a machine? Is there an error message? Are you able to select the machine within the task target?
Do you have a task server configured and does the client show it is registered with the task server in the Task Status tab of the agent? Any errors showing the client Logs or Server logs related to the task you tried to send?
Depending what "type" of task you are sending, some tasks require specific agent plug-ins such as Software Management Solution, Deployment Solution, Server Inventory, etc. You can enable the automatic install of those sub-agent under Settings --> Agents --> All Agents.