On 1098 form where is the certificate number located?

If this 1098 is reporting interest paid on property that was awarded a "Mortgage Interest Credit Certificate" you would have gotten a "Mortgage Credit Certificate (MCC)" from the state when you purchased the property. The number would be on that certificate.

If you DID NOT get any sort of state or local certificate to assist you with a loan to purchase a home, you would not have a MCC and you most likely are entering the 1098 in the wrong section in TurboTax. 

If you DID NOT get any sort of state or local certificate to assist you with a loan to purchase a home, enter your 1098 under Mortgage Interest, Points, refinancing, and Insurance" under the Your Home Section.