What happened?
The Original Prusa MK4/S, MK3.9, MK3.5, and CORE One are equipped with the xBuddy board. Thanks to this, and the capability for firmware to check for the connected components, the printer can detect a firmware installed for the wrong printer version.
If the printer recognizes a wrong firmware version installed, it shows the message "To continue, please insert the bundled USB flash drive and install the [Printer version] firmware", with the detected printer model shown in the message.
Error name: Different FW Required
Error code: #31701 (CORE One) #26701 (MK4S) #13701 (MK4) #21701 (MK3.9) #23701 (MK3.5)
How to fix it?
Download the firmware, and ensure the version matches your printer model.
Transfer the firmware file to your USB drive. Make sure to unmount the drive from your operating system before physically removing it. Then, follow the procedure for firmware flashing.