Cancelled plan and am still billed with late fees.

They do not pro-rate your final bill.  If you cancel even one day into a billing cycle, you have to pay for the full month.

For example, your billing cycle ran Nov. 14 thru Dec. 13.  You cancel on Dec. 1.  You still pay the full month.  No pro-rating.

This 👆. It's in the terms of service you agreed to. Section 5.1, there is no proration for any Cellular, TV, or Internet carrier, not just AT&T.

https://www.att.com/legal/terms.wirelessCustomerAgreement-list.html

I understand agreement except I had paid in full, the representative had credited the money back to my account for a months charge and I am getting a bill for this month that I cancelled. It’s not a prorated charge, it’s a charge for an entire month after I cancelled. If there was an issue the representative should have taken care of it. I clarified three times that everything was canceled and I would not be recharged for anything. Very poor customer service.

I have gotten two bills since I cancelled on December 1st and one with a late fee. My credit score has taken a hit and I am very upset. 

If you cancelled on December 1st, this would not affect your credit score. It will not hit your credit unless it is written off and sent to a collection agency.

Since December 1, are you getting one bill with added late fees? Or 2 bills with late fees?   

1 bill makes me think you were into a new bill cycle.   2 bills would indicate they didn't cancel service. 

I don't see how you would have 2 full bills since December 1, unless you were already into a new bill cycle at the time.  I believe you paid off November. 

You will have to call and speak to loyalty