The most common reason that Amazon is charging you for prime but you don't have it is you may have accidentally signed up for a free trial of Amazon Prime and it automatically renewed.
Someone else may also be using your account or there can be an error with the billing information.
If you notice a charge for Amazon prime on your credit card or debit card you should look into see if you actually are signed up for Amazon Prime and if so you can cancel it.
If you signup for the free trial it requires your credit card or debit card on file.
While you're not charged right away for the Amazon Prime free trial you will be charged automatically when it comes time for renewal if you don't go in and cancel the Amazon prime membership.
Another reason for Amazon charging you for prime is that your credit card or debit card details for compromised and someone else is using your credit card or debit card on another account for Amazon Prime.
Or someone else with access to your Amazon account such as a family member may have activated Amazon prime without your knowledge.
Or you could've accidentally signed up for Amazon prime during the checkout process or when trying to access a free trial that automatically transitioned to a paid Amazon Prime membership without you realizing it did so.