Problems in assigning Serial I/O to a PC COM Port

I am working on a project that makes acquisition of signal from a GPS via serial port.I am trying to debug my code following the example of the pdf Getting Stared:
MODE COM1 4800,0,8,1
ASSIGN COM1 <S0IN >S0OUT

I get the following error:
error 30: undefined name of virtual register
I cannot understand the reason of the error.Can someone help me, please?

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