Can someone help me please ? I turned the Use-Editor to true instead of false, it's supposed to work...

It is not supported to create quests on a server or a lan world (which is a server as well). This is due to limitation regarding syncing between clients.

It is not supported to create quests on a server or a lan world (which is a server as well). This is due to limitation regarding syncing between clients.

Oh...So I can't at all ? That's too bad, it was the only mod available on curseforge...

Not in a multiplayer setting, no. But it should be usable in singleplayer.