It's not uncommon for longtime customers to find that they cannot add sub-accounts, even though they have deleted one or more to make room. Here's what's happening.
When we change our pricing, we allow existing customers to keep the sub-accounts that they already have, but the new limits are applied for any new sub-accounts that they want to create. This is why you see the message about not being able to create sub-accounts. In cases like this, we would really like to be able to add back in the sub-account you deleted, but unfortunately the system doesn't allow it (because it is enforcing current limits). The limits are enforced at the account type level, not at the level of an individual account, so we are unable to make an exception for your account (even though we would like to!)
The support team has made a feature request so that customers will be warned of this when they try to delete a sub-account. We're sorry for the confusion!