launcher seems to "forget" which java version it's using
I often get an error message when launching ManyMC regarding my java version "not working or went missing", even tho i never touch any of the java VMs i have. I just have to re-select java 17 as the default version and it works fine, until a couple hours after closing it or after a reboot. little bug, but still annoying
Have you tried manually browsing for your Java and using its full path, instead of selecting the first result called just "java"? Maybe that's the problem.
This looks to be an issue with upstream MultiMC too.
This just happened to me today. Strangely enough, after I selected the same Java version as before, it doesn't prompt me anymore.