Hi Memos,
It is normal to have some background hiss in the system, but the guitar signal is always much louder than the noise, in fact when you play the guitar you should not hear any hiss.
I did the led screen and the blink tests and uploaded all the codes and it seems that the pedal recognises the effects.Also signal generator is working fine. Buttons are working fine too...but still no guitar sound..
If the signal generator is working thats good, because the output stage is working. So probably your issue would be located on the input stage.
Did you check the voltages on the op-amp? anything out of specs? you should have something like:
Im sure that you already did it, but did you tried adjusting the VR1 potentiometer?