In digital ocean, I believe you get a volume for free with your $5/month instance/droplet (it's not added separately, which differs from most other cloud providers).
The screenshot you shared is attempting to add additional volumes to a droplet. See the pricing for droplets, it includes 25GB of SSD and 1TB of transfer.
Edit: You're talking about spaces[1], I'm talking about volumes[2].
[1] https://www.digitalocean.com/products/spaces/
[2] https://www.digitalocean.com/products/block-storage/