In Camtasia 2018.0.4 it is possible that when the play button is pressed, the timeline does not play. Dragging the playhead will go through the preview, however, the play button will not play.
This is a known issue that is fixed in Camtasia 2018.0.5 and later. Select Help > Check for Updates in Camtasia to install the latest version. If updating is not possible, some customers have stated this can be caused by certain audio devices set as your default playback device, such as wireless or bluetooth headsets. Disconnecting those headsets can allow Camtasia's play button to work.