If any world is open while minecraft is changed, the world will say "Errors in currently selected datapacks prevented the world from loading. You could either try to load it with only the vanilla datapack "(Safe Mode)", or go back to the title screen and fix it manually." If the world has a datapack, a mod, or is on the fabric or forge version of the game it says this. The only time worlds open is if it is completely unmodded, and on the default launcher settings. The image isn't mine, but it is the same thing.