Hello, for some reason I'm unable to launch ATM9 from PolyMC, other packs work well I've tried multiple version and nothing works.

https://mclo.gs/y6OIiMv

Is it related to PolyMC or ATM9?

EDIT: Just to be safe I tried JDK 19 and JDK 17 too, they gives me this error while JDK 20 just isn't compatible and refuses to start. I'm pretty sure this eliminates any potential Java issue being the cause.

Try Prism Launcher maybe?
Also, what Java distribution are you using? Zulu JRE?

Oh never heard of Prism Launcher, let me try, I use the official JDK from Oracle.

Well turns out it works with Prism Launcher and JDK 17 Thanks a lot!