Well that kind of error is what happens when its still using the broken old forge maven jar’s, but that should be fixed with the prior command (would replace with GT6’s maven), so if it’s still happening then that’s very curious, it works for every system I’ve ran across, across a whole lot of projects that now use the GT6 maven, so if something is borked it would likely affect a lot of people, so I’m really curious as to the cause…
You could try deleting your ~/.gradle cache directory and then run that above refresh dependencies decomp command above to regenerate it properly and see if that helps?