TimeMachine cannot be found as the backup location
I'm trying to backup my MacBook, Late 2009 (mac OS High Sierra, V 10.13.6). I have always used TimeMachine with no external drive attached. Now it seems that I've done something and possibly removed TimeMachine as the backup location. I've attempted to set up TimeMachine again; however, it is not being found. Any suggestions at all? I'm at a total loss here! Thanks!
[Re-Titled by Moderator]