This is most likely because you are submitting using a type 2 (organizational NPI) as the rendering NPI.
The rendering NPI is the NPI that you have entered for the therapist under Providers in the top navigation bar of Therabill. The rendering provider NPI should always be a type 1 (individual) NPI. Your group NPI (type 2 NPI) goes under Admin in Therabill.
How to fix?
You need to edit the rendering provider in Therabill. This can be done directly from the EDI rejection page by clicking the blue pencil next to the rendering provider (not billing provider) on the right hand side of the page. Alternatively, you can edit the rendering provider by putting your mouse over Providers in the top navigation bar of Therabill and then clicking on Provider List in the sub-navigation bar. Find the provider in your provider list and click on the edit icon associated with the provider in your provider list.
Remember to save the provider after you change the NPI.
You can resubmit the claim directly from the EDI rejection page in Therabill by clicking the Resubmit Electronically button at the top of the page.