Backups & restore points · Where backups go
What does "refused to store the backup" mean?
Short answer: That cloud account won't accept the way Data Backup stores files, so the set can't back up there. Choose another drive or a bucket-storage account such as Amazon S3 as the destination.
It means that cloud account wouldn't accept the way Data Backup stores files, so the backup can't run there. The message reads "… refused to store the backup — this cloud account doesn't support the kind of writes Data Backup needs."
Nothing on your Mac is affected. To fix it, choose a different destination for that set: another drive with Choose a folder…, or a bucket-storage account such as Amazon S3, which handles backups well.