what is wrong with my bit register?

[Peste_Bubonica]

idk why it keeps me giving compare error

    PARTS:
    Mux(b=dffout, a=in, out=muxout);
    DFF(in=muxout, out=dffout);
    Not(in=dffout, out=notnot);
    Not(in=notnot, out=out);

(I put two not gate together because I don't know how to use the chip output as input)

[WBahn]

How does the Mux know which input signal to send to its output?