Value of element N403 is incorrect. Expected value for ZIP Code is 9 digits. Segment N4 is defined in the guideline at position 0300. Invalid data:

The billing provider on all health insurance claim must use the 9 digit Zip Code (also known as Zip +4). 

First, determine where Therabill is pulling your billing provider information from.  By default, it will be the information that you have on file under Admin -> Member Info in Therabill.  However, if you have set up any Groups or Insurance Company Specific Billing Provider information then you will want to check there to make sure the Zip code is entered correctly.

Another possibility is if, in the Advanced insurance settings for this insurance company, you have it marked to use the rendering provider as the billing provider.  If you are going to use this option, then you must enter a 9 digit Zip code for the rendering provider (under Providers) in Therabill.