Can't watch DVD with Linear Series

[zmlivingston]

I hope this message finds you well. We just purchased a 2017 Salem Cruise Lite model # 273QBXL. For what I gather the Linear Series radio FM/AM/CD/DVD is models RV4500. Unfortunately the DVD is not working. The message is " unsupported format" . I'm not using aftermarket DVDs, but I can't get it to work. Is there anything else we need to do, or forgot to do? Thank you.