I had this problem but failed to find a quick answer so I clicked the little explanation link and that had my answer :p
VirtualMin web hosting server backup error: “Failed to save scheduled backup : Deletion of old backups is only supported when a date-based destination is used, and strftime substitutions are enabled”
You can easily fix this by having your backup path and “file name” correctly formatted like this:
It wants a properly formatted file name with variables that it’s looking for, if you click on the text with the explanation link: Do strftime-style time substitutions on file or directory name
You’ll see what it’s looking for:
%d The day of the month as a decimal number (range 01to 31).
%m The month as a decimal number (range 01 to 12).
%Y The year as a decimal number including the century.