Why can't I delete a backlog list?
To delete a backlog list, first make sure it doesn't contain any backlog items. If you don't see any backlog items, select "All items" in the "Show" dropdown at the bottom of the Backlog tab, to make sure all backlog items are shown, including completed backlog items. If there are any backlog items, move them to another backlog list, or delete the backlog items.
If you still can't delete the backlog list, it's because it's the only backlog list in the backlog. A backlog must always contain at least one backlog list. So, add another backlog list first, or delete the entire backlog instead.