The payer is rejecting stating that you have entered the same modifier more than once on one or more of your service lines.
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 and they are not duplicated.
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.