Datapack error in the world

Basically I get an error or a  bunch of them when removing the data pack for a new version as the other side can't be in 1.21.4 it refuses to launch it makes me worried that this can't ever be fixed and thanks to this datapack use I'm stuck with 1.20.4 forever as this is in Aternos on a multiplayer if I try to find the sources like the items how? My friend and I scatter things around every where