Whenever i am trying to download backup file from admin, the file was corrupted with the error message on winrar “The archive is corrupt”. Is there any fix for this issue ?
Backup of what? Could be your FTP program is not in binary mode or many different reasons why copying a 'tgz' file from your server to your local system could be causing corruption.
If you're talking about database backups via cs-cart admin panel, then suggest you pick a table like cscart_access_restrictions, back it up, then restore it from the cart. If that's successful, then the problem is not with the backup but rather with how you are transferring the file.