I believe the issue is a simple as passing a non-numeric value to the mcw variable:

mcw = list(PO3_Ex = levels(us_data$PO3_Ex)))

Check to see what the output of list(PO3_Ex = levels(us_data$PO3_Ex))) is and make sure its the data type process expects which I believe is a numeric data type since its trying to use the trunc() function against it.