The EDI standards state that If the rendering NPI is the same as the billing NPI, then the rendering provider NPI loop is to be left off of the EDI claim and the payer is to consider the rendering provider to be the same as the billing provider.
Most likely reason for this rejection
If you have a type 2 (organizational) NPI entered for the rendering provider and the billing provider then only the billing provider information (and NPI) will go on the claim. The insurance company may receive this and when they apply the billing NPI to the rendering NPI, it will not be valid because it is a type 2 (organizational) NPI. A rendering provider should have a type 1 (individual) NPI.
How to fix
Edit the rendering provider (Providers->Provider List) in Therabill and make sure you do not have a type 2 (organizational) NPI entered for the rendering provider. You should enter the providers type 1 (individual) NPI.
Once you have made your updates, you can then go back to the EDI rejection page in Therabill and resubmit by clicking the Resubmit Electronically button at the top of the EDI rejection page.