93 - Entity is not selected primary care provider. Usage: This code requires use of an Entity Code.

The rejection is mentioning that the entity is not the primary care provider.   They do not specify the entity.  Most likely, the entity is either the rendering provider or the referring provider that you have put on the claim.

Referring provider

They could be saying that the referring provider you put on the claim is not the primary care provider for this patient.  Some insurance plans require referrals directly from the patients primary care provider that they have on file with the payer.

Rendering provider

They could be saying that the rendering provider (therapist) you have on the claim is not in their system as an approved provider for this patient.

You may also want to check...

You may want to call the payer to find out exactly what is wrong.  In rare cases, payers may require special identification numbers (a.k.a. legacy numbers or claim notes) for your therapist.