For the XPS 8900 drivers, firmware, Dell has stopped providing drivers, fixes, or security patches support for that system since year 2020.  For any warranty extension support, Dell also ceased to provide any warranty support due to the system end-of-life status.

If you wanted your service tag to be validated, you must contact Dell and provide proof of purchase as being requested previously.  Although, it is pointless to have it validated at this time.

About 6 more months, in October, your official operating system, Windows 10 will reach end of support by Microsoft.  To keep the machine running without any security updates, you need to figure out how to support the machine yourself.  We all do when needed to.

@Chino de Oro​ wow! Sad to see support has not improved. The proof of purchase was within my Dell account but that seems to be gone too. I had all my Dell systems in there going back to 1998. This service tag thing seems to be an issue with Dell as there are a lot of posts about it pertaining to the XPS 8900. To speak to any customer like this, with zero courtesy is making me consider going another direction with computer purchases. I make them last and continue working well beyond their expected life, often to show clients the durability of a Dell. With the issues I have experienced with my Dell account and this system, I won't encourage my clients and friends to buy Dell anymore. I provided proof of purchase when I first reported it, and still did not get my service tag validated, after it worked for some time prior to mysteriously being invalidated and not found. That is an issue on Dells end, as is the quality of their customer support. Take care, you just lost a long time Dell supporter. 

It sounds like you have mistaken my help and combining with your lack of understanding to form a judgement.  Am just a computer user and a community contributor, not working for Dell.

I pointed out to you about the facts that your 2015 computer is no longer received support by Dell and soon won't be supported by Microsoft.  Not supported does not mean it stops working.  With some learning and you can either add security utility and keep on using Windows 10 or installing Windows 11 in unsupported mode and keep your machine in service for some more years.

For whatever reason that caused your tag issue, it can only be resolved by Dell.  Although, you could have resolved it when you installed a new board.  But, with my knowledge, I told you that it's pointless.  As the tag does not provide any meaningful support at this time.

Just like other Community contributors, I have been providing support for free and there is no need to talk down at our effort.