A working node is the easy half. The skill that matters on the bench is reading a symptom and naming the cause. Each card is a real way the build goes wrong. Pick the most likely cause; the wrong answers are the other faults that look similar, and learning to tell them apart is the point.
You can now read the common symptoms back to their cause: a floating input, ADC2 with WiFi, a missing pull-up, 5V on a 3.3V pin, a reversed LED, a strapping pin, back-EMF, a baud mismatch. That is most of what goes wrong on this build.