Hi @Robi_Sen, welcome to the forum 
ArduSub is the firmware for the Pixhawk autopilot, which is different to and separate from the Companion Computer Software that goes on the Raspberry Pi.
Your picture also displays that you’re using a Raspberry Pi 4, which currently isn’t fully supported by our companion software. At the moment we only fully support Raspberry Pi 3B, not 3B+ or 4, but there’s a relatively major update coming in the next few months that will add full Raspberry Pi 4 support.
In the meantime, it is possible to use an RPi4 with the current software, but it requires some additional effort to set up, and there are a few issues with the installer script so some features may not be available. If you choose to continue with your RPi4 you can flash the SD card with a clean Raspberry OS Lite install, and then follow the instructions here to install the companion software.
Once you’ve got a Raspberry Pi with a working companion image on it I’d suggest you follow our software setup guide