The letter you sent to me didn't have a signature of your president A, which should have been signed in ordinary situation. Would you please send a letter with A's signature again? Thank you in advance.
The letter you sent to me was missing a signature of A, the president. Please resend a letter with (Mr. /Ms.)A's signature. Thank you for your immediate attention to this matter.
Could you please send a letter with Mr. A's signature on it as there was no signature of his on the letter you already sent me and where it should have been. Thank you.
すみません、1行目のpresidentとonの間に「A（社長さんの名前）」を入れてください。