If you look in the Modedefs.bas file or the "Serout" section of the PBP manual you will see that N9600 is defined as

N9600 = 6 ' Driven Inverted

Do either of the following

N9600 CON 6 ' Driven Inverted to declare the constant

or

SEROUT PORTA.5,6,["Hello World"]

Both should compile as you arent using any include files.