I started the world with only acacia data pack, is it possible to add another data pack to this world? Because there is no way to get lava, I got over 1000 gifts and nothing. please help.

You can enable the datapack by running the command

/datapack enable "carpetskyadditions/skyblock" before "carpetskyadditions/skyblock_acacia"
/datapack enable "carpetskyadditions:skyblock" before "carpetskyadditions:skyblock_acacia"

In singleplayer without cheats, you need to open to lan in order to temporarily enable commands.

For reference, does the order in the datapack selection screen determine which is loaded first?

the command only worked on /datapack enable "carpetskyadditions/skyblock" after "carpetskyadditions/skyblock_acacia"
she didn't want to activate on /datapack enable "carpetskyadditions/skyblock" before "carpetskyadditions/skyblock_acacia"
could it be a problem for my world?
When i entered your command it threw me out: Unknown datapack carpetskyadditions/skyblock
dont know whay

Sorry. The correct command needs colons, not slashes.
/datapack enable "carpetskyadditions:skyblock" before "carpetskyadditions:skyblock_acacia"

@razgriz5000 The datapacks higher up in the list are enabled after, meaning they have higher priority and override those lower down. "Acacia" needs to override stuff in "SkyBlock" so it should be higher in the list.

ok tnx allot its working now