Dell uses this support matrix from Nvidia to explain why it's not supported: Resizable BAR support    Now I can tell you first hand that a Ryzen 3000 CPU supports resizable BAR just fine, as I pulled my 3700X from my R10 board and put it in a retail board, and resizable bar worked just fine. Now as to why it's not supported on an R10 with a Ryzen 3000 series CPU, is because of the AGESA version on those motherboards. Dell has never updated the microcode that is required to support resizable bar.  That is why it works on a 5000 series R10, because they have a more recent AGESA version.   I gave up on trying to find out why they cannot update the AGESA code on the 3000 series long time ago and switched to a retail board. I actually just updated the BIOS on my motherboard the other day, which also came with the latest AGESA code ComboV2PI 1208.
I've tried everything I thought of and looked up, updated bios ( bios looks old with none of the options to enable SAM)...  my rig is linked here, am I able to have SAM on my PC....can't find a answer if my motherboard supports it....ANY KIND OF HELP IS MUCH APPRECIATED 
Aurora R10, cannot enable smart access memory