@ Kostas22, I have replied to your private message requesting for some details. Please share the same.    
Hello, I trying to add the new Dell Monitor which I just bought and I am getting the error "MPL Asset returns NULL value and unable to insert service tag." In the support I enter the Service Tag and it shows me, but when I press the "Add to my Products" I am getting the error. Thanks!
MPL Asset, NULL value, unable to insert service tag, #5