You can increase your available storage space by...
Upgrading your Pastela Plus plan
Upgrading your plan will provide you with additional cloud storage capacity.
For cloud storage capacity by plan, check the Plans page.
Logging in (linking) with a pixiv account subscribed to pixiv Premium
With pixiv Premium, your available cloud storage capacity increases.
For storage limits by plan, check the Plans page.
Deleting canvases you no longer need
Canvases in your "Deleted recently" still take up storage space.
If you have any canvases there that you no longer need, delete them permanently to increase your available space.
To learn more on how to delete canvases, please see below.
How can I permanently delete canvas data from the cloud?