when launching the game it crashes while loading.
https://mclo.gs/pY8tlKo

I had the same issue, the issue is with the java version being used. Double Slabs only works with JDK21, but 1.20.1 uses JDK17. I bypassed the issue by disabling java incompatibility checks but it would be nice if the mod itself was fixed so that it worked with JDK17.

i was already using java 21 and skip compatibility checks but it still would crash on start up load

I think I've finally got to the bottom of this - it was a config setting I missed, apologies!