I'm currently testing 18.07.70 (alpha release), and the recording of just audio (e.g. for voice over) is still not working.
In the recording menu there's two buttons for video and audio, but if one selects audio only (e.g. to record a voice over as a vocal comment), it just issues an error message.
Former analysis of this problem with older versions showed, that kdenlive still want's to open a video camera, even if not selected, and then aborts, if there is no camera.
It’s on the todo list: https://phabricator.kde.org/T9292
With version 18.12.1 screengrab with audio is fixed. Please have a look.
Voice over is on the task list: https://phabricator.kde.org/T10314
*** This bug has been marked as a duplicate of bug 367676 ***