The Original Prusa MK4/MK3.9 is equipped with a Power Panic feature. This feature can recover a print in case of a sudden power loss (read more about it here). The power panic cable goes from the printer's PSU to the xBuddy board. In case there is any damage to the cable, the printer will show "Power panic has been detected during printer initialization. Inspect wiring of PP-cable."
Error name: Power Panic
Error code: #13321 (MK4) #20321 (MK3.9)
Check the connection of the cable both on the PSU and on the xBuddy board. Make sure that the cable is not damaged.