This payer appears to require that you register each of your rendering provider NPI under your billing provider NPI that you have on file with them. Most payers do not require this, but some do (for example, Blue Cross Blue Shield often requires it, depending on your state).
The rendering NPI is the NPI that you have entered for the provider in your provider list in Therabill. On a insurance claim form, this NPI goes in to Box 24J. You can get to your provider list by clicking Providers in the top green navigation bar of Therabill (when logged in to your administrative account). Here, you will see each of your therapists (rendering providers). Click the blue pencil icon next to the therapist to see what you have entered as their NPI.
The billing NPI is the NPI that you have entered under Admin in Therabill (unless you are using Groups or Insurance Company Specific Billing Provider information). This is the NPI that goes in to Box 33A on the insurance claim form.
What you need to do
You should call the payer to find out what they need you to do in order to get your rendering provider's NPI covered by your billing provider NPI in their (the payers) system. Once they have told you that they have registered your rendering providers NPI with your billing provider NPI, you can then begin submitting claims to them.
Resubmitting a claim that came back in error with this EDI rejection
In Therabill, go to your dashboard. On the right of your dashboard, you will see a section labeled Electronic Submission Errors. Click on the error within that section to go to the claim error page. At the top of the claim error page, you will see a button labeled Resubmit Electronically. Click that button to resubmit the claim.