You did not say what version of Ubuntu MATE you are using.
Based on your statements in this post it appears you are seeing the gpg keyring server error. The last closed issue on micahlee's github was in October 2019 so I'm not seeing any issues you have opened that were closed recently: https://github.com/micahflee/torbrowser-launcher/issues?q=is%3Aissue+is%3Aclosed
The author of the torbrowser-launcher program states on his github page:
"Are you getting an error? Sometimes updates in Tor Browser itself will break Tor Browser Launcher. There's a good chance that the problem you're experiencing has already been fixed in the newest version. Try installing from Flatpak (instructions below), or build from source."
On the bottom of this page he offers instructions for installing via a PPA or a flatpak.
I am including his PPA instructions for brevity:
sudo add-apt-repository ppa:micahflee/ppa
sudo apt update
sudo apt install torbrowser-launcher
Good luck