This error indicates that the PC's upgrade tool is waiting for a response ("Get" command) from the STB, but the STB is either not sending it or the signal is being lost due to connection issues.
If you are still getting "Please Check Stb Uart Receive," you may need to open the box.
# Check which serial port ls /dev/ttyUSB*
Ensure you are using a (cross-line) cable if required.