Which version of NPO are you using ?
There were some bugs, but in any case: the “Nerr” is not due to TRE !
It is due to one of three things:
1/ PM Files are corrupted
2/ The value of the indicator is a rate : X / Y, but Y = 0. So the indicator = Nerr (you can’t divide by zero 🙂 )
3/ Bug in NPO