If you are attempting to delete an organization from its Settings area and are receiving the "You must belong to at least one organization." error message then this means it is the only organization you are a member and the system requires that you be a member of at least one organization so it will not allow you to delete that organization.
Assuming that you have already deleted all existing databases within that organization and your account is all paid up as well with no unpaid or pending invoice amounts you should be able to delete the organization by deleting your entire user account via the Account Settings area:
https://app.planetscale.com/settings/account
Do keep in mind that account deletion utilizes a double-confirmation process so you may be redirected back to the page above and have to press the "Delete account" button a second time after an initial account confirmation step.
If you have any pending invoice amount present for the current month and that is preventing you from deleting your account you may go to your organization's Settings area and scroll down to the "Delete organization" section and expand the "View checklist" area and if the steps shown there are completed you should be presented with a button to pay the current invoice amount early, rather than needing to wait until the end of the month when invoices are processed automatically.