1. Work/school accounts with full Teams subscription, business/enterprise. 2. Personal accounts (Teams for personal use), limited features. 3. Teams free (Teams free organizations) including some business features.
When deleting a Teams free org. you don't delete your account but the organization. Once again, did you create a Teams free org. with teams and members in it? Then it's Teams free.
You don't delete a Teams for personal use account (also for free). You delete the associated Microsoft account.
So the question here is if you have used the same address for your Business Basic subscription which includes Teams as for the one you want to delete, whatever service you signed up for, and that will decide how to proceed. Also, important to use InPrivate windows as you might noticed.
So I had an existing Microsoft account. I created two free teams accounts - one personal and one business. I will keep the personal one but want to delete the business one, which was set up with the same address I have since used for the business basic account.
You should be able to follow the guide for deleting a Teams free organization then, really important being InPrivate mode with your browser. If no success reach out the Microsoft support, details in the link already provided.