First be sure it is a legitimate letter from your bank. There are a lot of scams out there. So verify the letter with your bank.
This is mainly a user community forum, so we won't know anything specific about your bank and letter. You should call or visit them for any questions about your letter.
Normally when you open a deposit account, in addition to proof of I.D. you have to provide your Taxpayer ID Number (which for most people is an SSN, or EIN if a business), and sign a statement certifying whether you are subject to backup withholding or not. It's possible that the statement about withholding is the "certification" you mention, but we won't know here. Is the bank saying that they do not show that you have provided both of those items when you opened the account?
If you DID provide all that when you opened the account, then I would first wonder if the letter is real. Again, phone your bank or go in person to verify the letter and see exactly what they want and why they didn't have it in the first place.
