When you are downgrading from a paid plan you need to check 2 things:
Free plan does not allow any private projects and dataset.
So you need to convert the projects and dataset to public . Alternatively you can also delete them. You can do this from the Settings tab in the project / dataset page.
Free plan has a smaller disk storage limit per account (10GB).
So you may need to delete projects(or jobs) and dataset (or dataset version) to keep your storage under this limit and try downgrading again. Alternatively you can purchase Storage powerups if you would like to retain your work in the free plan.
Note: You will get the same error even when you will try to remove a storage powerups and your Storage will exceed the Free Plan quota.
Team plan cannot be downgraded from the website.
Contact support if you would like to convert your team account to an individual account.