Mux16(a=false, b=y2, sel=f, out=y_add);
Can't connect part's output pin to gate's input pin: load ALU.hdl
'_' is not a legal character in HDL names. You want to change "y_add" to "yAdd".
Why the strange error message?
The parser saw this as "out=y". "y" is the "gate's input pin" in the message.