Service Tag not recognised

I've a friend who gifted me A Dell T5500 Precision Dual Xeon CPU tower. He had no idea of specs nor condition. I opened it had a look...it had 3 ram modules for both the Main Board Ram Slots and 3 for the Supplementary CPU Slider Ram Slots.

I plugged it in [no monitor nor peripherals] just wanted to see if it turned on...beeped for POST.

It turned on, then gave 5 beep error {guessing that's for no monitor detected?].

Took a couple of photos and when I got home, logged into Dell to search for info...using the Service Tag on the tower...got the result; "There is no match for that Service Tag or product ID. Please try again, auto-detect or choose from all products"

The  Service Tag sticker is not damaged, I read it correctly and typed it into the search bar correctly...why am I getting no results?

I'd really like to find the original specs of the unit...to guide me as to upgrade possibilities and whether I make the effort to refurbish the tower...