I am having recurring play testing issues

Whenever I try to test my mods by opening a forge world from inside of MCreator, when I try to log into a world that I have already tested and played in, an error pops up saying, "Errors in currently selected datapacks prevented this world from loading. You can either try to load it with only the vanilla data pack ('safe mode'), or go back to the title screen and fix it manually". I am not sure what to do in this situation, as when I click the "safe mode" button, nothing happens. Trying to create a new world instantly takes me to a screen saying "Saving world" and closes the loader. This has been happening for a few days now, any help is appreciated.