It seems there is no USB communication between EDU09 and PC.
Did you check the voltage on pin 20 of IC3?
If voltage OK, please try other USB cable if possible.
OK, guess I found it: Left from the notch is pin 1, then counts counterclockwise, right? So right side from bottom starts with 15, then up 16… 20 measures -3,36 V.
Tried four different USB Cables - no effect. 5 V enters the board from the USB Plug.
You are right how to count the pins.
(The voltage on this pin should be positive compared to GND.)
Anyhow, the voltage value 3,36 looks OK.
Please check the connection between IC3 and USB connector:
Check that there is connection between the USB connector pins 2 and 3 and pins 15 and 16 of IC3.
Check also that there is no short circuit to GND and no short circuit between the pins 15 and 16.
Don’t know the numbering of the USB Plug, however, see drawing:
The two upper (inside the board) Pins have 5V between them. From the lower / outer Pins, the Right one connects to 16, the left one measures 500 kOhms to pin 15. Resoldering did not help.