The Original Prusa MK4/MK3.9 firmware is in BBF format. When flashing firmware, this is copied from the USB drive to the MK4/MK3.9 internal memory: in case of an issue in allocating the internal space for the firmware file, the printer will show the message "Space allocation for firmware BBF file failed. Repeat the action or try another USB drive."
Error name: BBF Allocation Failed
Error code: #13531 (MK4) #21531 (MK3.9)
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.
If the issue persists, attempt flashing firmware using any other USB drive in your household: make sure this is formatted to FAT32, copy both firmware files, and repeat the previously described procedure.