When opening the "MCU-Flasher" window, the message "CAN interface is already occupied by a program or not connected" appears even though the PEAK-USB adapter is connected

First, connect the PEAK-USB adapter so that it lights up red. If it is already flashing without "MCU Flasher" open, it may already be in use by another program, e.g. from the PCAN viewer. Check for the presence of the adapter in the Windows Device Manager.