Contact your seller for an explanation of why the service tag is not valid;  only they can explain.  If they cannot or will not assist, return the system for a refund.  
Awhile back I was looking for technical support and then my service tag did not work. So I called Dell and they told me it was a faulty PC and that they would replace it. But spoiler alert, they didn’t they said they need the purchase receipt. But I don’t have it because I bought it of eBay. They said they can’t replace it and my service tag does not work.
Dell won’t fix or replace PC bought from eBay