Error when writing command to switch off the projector

Hello all.

I need an urgent help in the project stuck since weeks. We have bought a projector and are trying to write a QT code to control the projector. We have used the QserialPort provided by QT as given:-https://code.qt.io/cgit/qt/qtserialport.git/tree/examples/serialport/terminal?h=5.13

However, when I try to send the command "WT +LEDE =0" to switch off the projector, it don't get turned off.
The program can connect to the COM3 serial port. but cannot send the command.

Any help from the community will surely help us in the long run.

Do you have correctly set transmission parameters (parity, word length, speed) ?
What does it mean : cannot send command ?