The printer is showing the message "Unknown error"
Error name: ESP Error
Error code: #13506 (MK4) #21506 (MK3.9)
On the Original Prusa MK4/MK3.9, the xBuddy can be extended with a Wi-Fi module. The error is triggered in case the printer detects an undefined problem with the Wi-Fi module.
The firmware needed for the Wi-Fi module is included in the MK4/MK3.9 firmware file. Download the firmware and if necessary the bootloader, and transfer it to your USB drive. Make sure to unmount the drive from your operating system before physically removing it.
Insert the USB drive back into the printer and reboot the printer using the reset button directly next to the knob to initiate the flashing procedure. In case the procedure does not start, reboot the printer again by pressing the same reset button, and while the printer is booting up, press once the rotating knob.
The printer will let you know when the flashing is done. When installing the bootloader along with the firmware, it may take several minutes.
Access the xBuddy board by loosening the four screws in the below picture, and inspect it for any visible damage.