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.