@fl41517
You didn’t provide any Appium logs or anything that really help, but I am gonna answer as much as I can.
XML DTD is disabled by default in all XML readers to prevent creating what is called an XML Bomb (Where you reference an already defined element in multiple levels and cause the XML reader to overflow the machine’s memory and crash it), Your website’s source is fetched correctly but, the xml tree creator has detected a DTD reference and hence the error.
This is not a bug in Appium and for security reasons this check should remain on. My advice to you is to refer to the website developers for help on changing the element that is causing the problem.