When blogger introduced new changes to blogger themes and controls, it also disables the adding of Html/Javascript in your "Add Page Elements" section in blogger without the corresponding Title. Hence, you get this message "Required Field must not be blank". See photo.
This was not the case before. We don't always need to put a title to the elements we put in our blogger blogs, hence to solve this problem just put this code in the title field "<h></h2>" See photo below and then save. Your added javascript should not contain any text in the title.
