CO-16 just means there is an error in the information submitted.
CO-16 Claim/service lacks information or has submission/billing error(s). Usage: Do not use this code for claims attachment(s)/other documentation. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT.) Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present.
There should be remark codes reported with the CO-16 that will provide additional guidance as to what the true error is and what needs fixed. (Ideally, your payment poster should have posted the remark codes along with the claim adjustment code if your system allows it.) You really need to see the full copy of the original explanation of benefits or to call the insurance company for more information about the denial. You're just taking a shot in the dark to guess at this point. For example, Anthem BCBS regularly denies using a CO-16 when they want medical records for the claim. Once you know more about why the claim denied, you'll get better help from the users on the forum.