U12 means the door is open in the Panasonic FR1 family. Check the visible latch when the unit allows opening; do not confuse it with a lockout due to water or temperature.
The FR1 chart identifies this state for the NA-V10FR1 and NA-S106FR1. A top-load setup may require a different arrangement and procedure.
If the cycle does not start
Check that the laundry is inside the drum and not caught in the seal. See whether anything is preventing the door from reaching its position, and close it normally. Do not use force or keep the latch pressed in an attempt to start the cycle.
If the door is already closed but U12 comes back, note that. The message by itself does not tell you whether it is a broken part or simply a missing confirmation.
If it appears during the cycle
Pause and allow for the unit's lockout period. Water, heat, or motion can prevent opening even if you want to check the edge. Do not force the handle or bypass the safety lock. With the outside area clear and the alert still present, request service with the reference and the point in the cycle.
When asking for help, distinguish between a door that does not seat properly and a door that seats but is still detected as open.