Backups program fails to backup. And the Backup Now button is grayed out

I had been running the Backups program successfully in Lubuntu 18.04.3, in order to backup the Home folder to an external drive (which has plenty of space on it).

The Backups program worked successfully for several months. For the last 318 days now, though, the daily attempts of the Backups program to perform backups automatically have failed. And the Backup Now button is grayed out. (Since then until now, I was too overwhelmed to get engaged on this web site to try to get help to fix the problem.)

I am a relative newbie - and almost always I use GUIs. I can perform Terminal commands IF they are provided together with complete and precise instructions.

My thanks to guiverc for the helpful comments and links, especially yesterday. Accordingly, yesterday I read within Deja Dup`s "Back Up Now" button is not highlighted how I could un-gray the 'Back Up Now' button.

Specifically, yesterday I killed a process id (that is, a pid) for the backup program DejaDup. It was the pid of DejaDup that ended in ' --backup --auto '. After rebooting the computer I found that the 'Back Up Now' button was un-grayed. Then, I clicked the 'Back Up Now' button, and DejaDup started creating a fresh backup, and the reason it provided was "in order to avoid corruption" of the backup. However, after about 4 hours the backup attempt failed, and then the 'Back Up Now' button was grayed out again. Today I used the command line 'killall deja-dup', and once more the 'Back Up Now' button is ungrayed.

Via the website for DejaDup, wiki.gnome.org/Apps/DejaDup , I have filed a support request with the Gnome Infrastructure Team (at GitLab). Whereas DejaDup was updated on 13th January 2021 (according to the Software utility in lUbuntu 18.04.5), and whereas the aforementioned website for DejaDup was last updated on 25th August 2020, nevertheless this problem of grayed-out 'Back Up Now' button (and lack of a progress bar, by the way) is clearly over 8 years old by now.