In my online medical history (mypennmedicine. Org), it says "resolved chickenpox" under medical history. I've never had chickenpox. Is this an error?

Not likely. You may have been tested at some time for antibodies for the varicella-zoster virus. If positive you have had varicella (chicken pox) at some time in the past.
Maybe. If you believe there is any errors in your medical records the HIPPA law states you can request that they be amended/corrected.