Heyho,
im trying to set up my own skyblock wolrd the first time. I checked if the data packs might have the wrong version (currently carpetskyadditions-1.19.3-4.2.0; cloth-config-9.0.94-fabric; fabric-carpet-1.19.3-1.4.96+v230201) and im playing on 1.19.3 of course. AS soon as I switch out the default Datapack with CarpetskyAddtitions and click on DONE it says Dara pack validation failed!
What am I supposed to do now? Or is it the wrong way to set this up?

The Default datapack is supposed to stay enabled. Just make sure the SkyBlock datapack is higher in the list and it will override the defaults.