Go back to channel 11, check versions, then try again with channel 12.
I'm having the same issue, also on Raspberry PI 4.
I can change channel from 11 to 12, but the "available versions" list is empty. I have changed back and forth multiple times, enabled/disabled custom channels, and rebooted.
Could this be related to the change of architecture from 32 to 64-bit? I.e. libreelec won't find any compatible entry in the releases.json?