I remembered the reason why I couldn't find the page about the validation field.
Because I Iooked over the documentation about validation, I fixed the validation error.
as a result of that, it did not appear the error of ",2,4,5" any more.
After fixing the validation error, I had to meet another error saying "Invalid information, please check your entries and try again".
So, I manually input a user information to phpMyAdmin through Insert item and I tried to register the registration page,
but the page did not work to carry my input to database which stored the same information registered by Insert item.