MODIFIER 0 IS INVALID

The rejection message "Modifier 0 is Invalid" (where 0 will be a number, i.e. "Modifer 2 is Invalid") is indicating that the 0th modifier you have applied to the service code is either 1.) not a valid modifier or 2.) they do not accept that modifier.

How to fix

The modifier is entered on the add / edit session form in Therabill.  Open up the edit session form for the session(s) on the claim.  On the claim, next to the service code entry text box, there is a box labeled Modifier.   Make sure you have entered valid modifiers in to that text box.

IMPORTANT: When entering multiple modifiers, separate the modifiers with a period (.).  For example  "GO.KX" would be two modifiers, GO and KX.  If you do  not follow this formatting, the modifiers will not go on the claim correctly.