First thing to check is that you have free web space available. If you've filled your web space we can't backup. The way the backup works is to collect and compress all your data into a backup file, and then you can download that file, or send it to our online vault.
If there is not enough web space for the backup file, it will fail like this. Unfortunately, it is not possible for us to detect available space for your account since all hosts differ in their measurement methods and most don't let our scripts see the available space.
Have you enabled the Backup parent WordPress folder? As looking at the path in our error message, you have enabled this, but looking at your WordPress paths, you shouldn't be using this feature. The feature is ONLY intended when WordPress is in a subfolder in your web space (which is inside public_html). Your WordPress appears directly to be in your public_html, and not in a subfolder in there. So I advise that be disabled since many hosts have various links, metadata, logs and backups above the public_html folder which shouldn't be needed in the backup and commonly cause issues.