struct paylaod_t {
	uint8_t len;
	enum commands_t command;
	uint8_t data[USB_RX_LEN-2];
};

extern volatile uint8_t *usb_rx;


int main(void) {
	int16_t i;
	volatile struct payload_t *payload = (volatile struct payload_t*) usb_rx;

.....
....
	
   switch(payload->command) {  <-- dereferencing pointer to incomplete type