It is possible that your WP site is running on a 32-bit system. 32-bit systems cannot process files over 2GB in size.
The backup file will exist. It just can't fetch the file size. However if you want to solve it -
Solution 1: Please move your WP site to a better 64-bit server.
Solution 2: If you want to stay with the existing server, please reduce your backup size by excluding big files / folders.