Comment # 10 on bug 1212486 from Kudera

I ran into this problem just last week. As for the others here, Deja-dup
produces the same error message about missing duplicity even though it is
installed.

For me, however, it works without complaining if the Yast Software management
is open. The software management is running with root permissions, and deja-dup
(hopefully) just with those of my normal user.

That's on Leap 15.5.