I keep forgetting about this snag so I’ll document it. In Debian / Ubuntu distros, once you’ve added the PPA to have the latest version of KeePass installed, if you try to install the KeePassRPC plugin (Kee) it will tell you the version is incompatible, even though it is.
The following plugin is incompatible with the current KeePass version: /usr/lib/keepass2/Plugins/KeePassRPC.plgx Have a look at the plugin's website for an appropriate version.
The fix, as found here, is to install the mono-complete package
sudo apt install mono-complete
Restart KeePass after installation. That’s it!