Nextion Display not getting updated data
recvRetCommandFinished err means something is wrong with the serial communication between the Arduino and the Nextion display; it is what recvRetCommandFinished() prints when there is an error.
Check if you are using the right pins (TX1 and RX1 for Serial1) for the serial connection to the display, the actual connections you made (RX1 to TX, TX1 to RX), the baud rate, and the cable.