This means that you already have a swapfile, it’s called /swapfile, and it’s in use.
If you want to make it bigger, you can stop your container, then unmount it, and then use that dd command to make it bigger, and then the mkswap command.
If someone else reads this, the way that I do it is to grep swap discourse-setup and look at the commands there. It’s a slightly easier version of what to do.