Make sure you have launched the game before putting the dlls in, just creating the folder manually wont work, Or you probably downloaded the latest bepinex instead.
Viewing post in Baldi's Basics Mod Menus comments
I downloaded the 5.4.21.0 BepInEx version. also I did not create the folders manually. And I did extract the BepInEx zip file, put it in Steam>steamapps>common>Baldi's Basics Plus , And I loaded it up and I pressed Alt And F4, and put the mod menu in BepInEx > Plugins , Loaded it back up, went into Hide-and-Seek mode, pressed Tab, And it did not work.