Simply put, on both a very newly-installed copy of Mint 21.0 on real hardware and the Mint 21.0 live ISO running in VirtualBox, I'm getting a "Public key not available" error when trying to install WINE.
Thing is, I've installed WINE many times and have even successfully installed it on Mint 21.0 in the past yet, for some reason, I'm running into this error now.
Again, this is even with a very fresh newly installed installation whereby, immediately after installing the OS, I went to install WINE. Similarly, when running the live ISO in VirtualBox, immediately after booting, I went to install WINE.
...yet both are running into the very same issue despite this exact process having previously worked.