It happened when villains(toads) were attacking an ally. Referring to the link below, I saved a new file and that worked(phew), thanks!
https://www.reddit.com/r/RPGMaker/comments/j781d0/need_help_with_error_message/?...
Also, whenever I start game, there's a message popping:
"Failed to load
img/menus/Background_01.png"
After I click "retry", the game works well but still kinda annoying. It'll be glad if it can fixed. Thanks.