The F28 error code signals a connection issue. You can open the top and unplug/replug connections. Make sure that none of them is loose. This method works in 75% of F28 error cases. Also, the code may appear when the bottom panel is removed.
Disconnect the washer from mains or switch it off for about 30 seconds.
Reconnect/switch on the washer and press Pause/Cancel/Stop to clear the screen
Run a cycle to check the machine’s operation. Select a short cycle, then press the start button and hold it until the cycle starts.