It’s somewhat difficult to translate, because Arabic doesn’t have the concept of case in letters. Usually you can use “حروف صغيرة” or ”حروف كبيرة” which literally translates as “small letters” and “big letters” when referencing other languages. For the general “letter case” you can use “حالة الأحرف”. So it’ll be something like : تجاهل حالة الأحرف.
So here you substitute الرسالة for the correct word الأحرف to mean “letters”