This is a short guide to help you out to find any bug on the hardware:
Common mistakes: 1. Are you sure you have an Arduino UNO (not Mega, not DUE, not any other)
2. Guitar connects to the right, amp to the left jack connector, like:
3. Electrolytic Caps placed wrong: Have a look to a good PCB in high resolution and check that you have yours in the right orientation.
4. Integrated circuits placed upside down: Its easy to place them wrong by error, the pin one with the small circular notch goes top left.
5. Check any short circuit between nearby pads and check that all solder points are good. To try to avoid dry joins you can re-solder them for a second.
Once all the things above are checked, the best way to troubleshoot the PCB is doing it in 4 steps:
1. Check the power supply: There are some test points on the PCB labelled as GND, +2.5 and +5V. Using a multimeter make sure that you have there the right voltages. Also check that the op-amp is correctly powered (GND on pin 4 and +5V on pin 8 ).
2. Check the output stage: You can load the sine-wave generator program. It only uses the output stage independently that the input is wrong. If it works you can be sure that the second op-amp area is good. If not check for resistors placement, IC is the correct one? orientation?
3. Check the input stage: Be sure that the trimmer VR1 is in a "medium" position, you can adjust it better later. Load a clean/volume pedal and check the same things as the step before.
4. Check your Arduino UNO board. Sometimes there are some dodgy PCBs that do nto work properly and can drive you crazy. There is plenty of examples if you go to the Arduino software and click on File/Examples.
Try to load codes to light the LED, read the pots, there is an interesting Monitor code in the forum that can help you with that.
As a tip I can to say that 99,9% of the errors are due to some connection is wrong or some component is misplaced. From previous experience building dozens of pedalSHIELDs they always work straight away from soldering correctly, but we are all humans and we all make mistakes.
If all the above fails, just carefully component by component that it is well placed and orientated, dont take anything for granted.
If can borrow another pair of eyes to have a look at it, do it. Sometimes others can be things that we are passing over.
If you have an oscilloscope or a signal generator (I use Visual Analyzer)