Can you try to deactivate all the plugins not related to WPML, activate a default theme (like Twenty Nineteen), and tell me if you still see that warning and can create a backup, please? This way we can make sure that this is not a compatibility issue between one of the plugins (or theme) that you're using and WPML.
1. Backup completed successfully.
2. I've deleted Porto theme, Child Porto theme and delete all plugin except wpml plugins.
See images attached (2,3,4,5)
So now that you deleted the Porto theme and all the other plugins, you managed to create the backup, am I right? I mean, this was the problem in the first place. Can you tell me if there's still any problem here, please? I mean, I still see an error message on one of your screenshots but I don't understand when is this happening or where I can see it.