So for this mod pack to work, it requires JAVA18 and from my experience you have to use what it specifically calls for. I installed a couple of days ago a forge modpack of Roguelike Adventures and Dungeons 2 and this one requires JAVA8. I tried it with JAVA17 & JAVA21 and it says “failed to start” every time. Once you install the correct version of JAVA, you need to restart AMP and then manage the instance to select the correct JAVA version. If you do not see it in the drop down, then i would recommend restarting your PC.