i got the same thing but never explored why or how to fix. I had ubuntu.podcast playing at the moment I got the error where they were talking about the "software boutique", so I gave it a try and presto - i had tor browser installed. if no-one else can answer as to a fix; you could always try my lazy-ass work-around
[software boutique is a new-ish [ubuntu-mate] ubuntu-software-centre that contains common or best-in-class type apps, skipping many less-used ones]
addendum: if trying this, un-install current version first; as I was left with two menu items & icons (one for fully-installed by boutique, and other failed by apt-get); so its probably quicker to have none-installed first instead of working out which is which to un-install the partially-installed-one..
gpg --version
, it has to be 2.2.17 or newer. – Alexey Burdin Oct 16 '19 at 09:35