[ping]

you need to read the user guide again and you will see that you can do exactly what you are asking within a region based on a wall set to be the transition wall - the image in there shows you the cells that are then suppressed

or you can also supress it in a whole region so to use this you need two of more regions which are interfaced to allow flow between them

you can only have one physics continua in regions that transfer flow - a bad error of yours

and of course you could always use the more advanced gamma re theta transition model which predicts where transition occurs rather than you specifying the location in the above approach