Don’t know where the mistake is. The users are not supposed to read the entire code before selecting the device and start using it.
The PinNames.h file shows the USB pins: mbed-dev - mbed library sources. Supersedes mbed-src. | Mbed
The PeripheralPins.c file shows USB pins: mbed-dev - mbed library sources. Supersedes mbed-src. | Mbed
The system_clock.c file shows clock selection for USB.