Currently, online filing of Form I-751 is not stipulated. According to the official USCIS website (www.uscis.gov/file-online) at the present day you can file online only the following forms: Form I-90, Form I-131A, Form N-400, Form N-565, Form N-336.
Meanwhile, you may be a little bit confused with Part 1 item 9 of Form I-751, which requests you to provide your USCIS Online Account Number (if any). In case you have previously filed a petition, application, or request using the USCIS online filing system (formerly known as USCIS Electronic Immigration System), you do have it, the USCIS Online Account Number was issued to you by the system back then. Simply indicate it in the space provided in Part 1 item 9. If you do not remember where to find your USCIS Online Account Number, you may find it by logging in to your account and going to the profile page. Also if you have previously filed certain applications, petitions, or requests on a paper form via a USCIS Lockbox facility, you may have received a USCIS Online Account Access Notice issuing you a USCIS Online Account Number. Your Number can be found at the top of such Notice.
Please do not confuse the USCIS Online Account Number with the A-Number.