Distributed rendering cannot be enabled during rendering. Please enable distributed rendering only after stopping the current rendering.

This message is triggered once the "Enable" checkbox is checked in Render Setup > System > Distributed Rendering while a local rendering is running. 

This is an unsupported scenario, and in such case the distributed rendering will NOT be enabled unless the rendering is restarted. It is currently not possible to toggle distributed rendering while rendering is running. 

The solution is to stop the current rendering, enable the Distributed Rendering checkbox, and then start rendering again.