96 - No agreement with entity. Usage: This code requires use of an Entity Code.

This rejection message from the payer is indicating that there is no agreement with the entity on the claim.  Unfortunately, they do not specifically mention what entity they are referring to.  More than likely, they are referring to one of the following.

• Your clinic is not registered as a provider for this payer.  Some payers require that you contract (or become in-network) with them prior to submitting claims.  Remember,  you must also submit claims to them using the billing NPI that you gave them when you contracted with them.
• It could be the the patient is not covered under this payer.  That is, the patient does not have an agreement with this payer.