Only the seller can assist.  Unless they have a satisfactory explanation, return the system for a refund, or accept that you have no support/warranty with Dell.  
Hello everyone, I just bought used/secondhand XPS 9500, The seller said it was a gift and that he never registered it. He was able to show screens of Support assist that showed it was originally purchased in October 2020 and had warranty until October 2021. After wiping it and reinstalling Windows  support assist says "Unable to show warranty details". I have tried to register and also transfer the ownership but it just says "Invalid Service Tag". Anyone have solution about this? Thank you
Service Tag Invalid