I mean, Minecraft 1.12.2 does work with Java 11, like you can use Java 11 on Vanilla 1.12.2 and Spigot 1.12.2.
But older PaperMC versions(1.16.4 & older) no longer support Java 11 because PaperMC released patches for the log4j exploit for older versions. These patched builds no longer support Java 11, which is why you can't select it in the panel anymore when your server is running on a version older than 1.16.5 of PaperMC.
To use the plugin, you either have to switch to PaperMC 1.16.5+ or Spigot 1.12.2+
Thanks! I changed my server's version to 1.16.5 just beacuse I need PaperMC.