Hey sabbir_nfs,
Thank you for using Apple Support Communities!
I see that you're having issues upgrading to macOS Catalina. I'd be happy to look into this.
Since the download is not completing, let's begin by checking the internet connection.
1 - You'll need to be connected to a Wi-Fi internet connection and not a cellular hotspot.
2 - Check to see if there are any reported issues with the macOS Software Update on the System Status page: Apple - Support - System Status
3 - Restart your MacBook by going to the Apple menu in the top left corner of the screen and selecting Restart.
4 - Try updating again.
5 - If it still will not complete the download you'll want to be sure to try on another Wi-Fi network.
Let us know what happens!
Cheers.