I don’t think the problem is necessarily from these two M-ary pulse generator, as mine always stops on M-ary pulse generator_6 and sometimes others. Besides, I omitted decision component and pulse generators 4 and 5 and still saw the error. I haven’t also managed to find the problem so far. however, you have chosen 16-QAM in decision component while you have used 4-QAM. You need to correct this anyway.