Hello carlosmp-SW2016,   I see in our internal tool, Customer Kit Selector that the processor 6248R is supported on the C6420.   PN#338-BVKI          Info: Intel Xeon Gold 6248R 3.0G, 24C/48T, 10.4GT/s, 35.75M Cache, Turbo, HT (205W) DDR4-2933, CK   Sales may have other options if you would like to contact them.    
Hi, I was looking at the specs of the C6240, and it seems to support CPUs with 205W TDP and 28 Cores.  I have found 2 different technical guides, one that only has Gen 1 Scalable, and another with only Gen 2 Scalable.  Is there any reason that the 6248R is not listed?  It's a 205W TDP and 24 Cores.  Only thing I can think of is that those along with several others (6240, 6250, etc. ) were all introduced Q1 20, and therefore not included?   I've looked at BIOS update docs, and don't see anything CPU specific Dell EMC PowerEdge C6400 and C6420 Technical Guide poweredge-c6420-spec-sheet.pdf (dell.com) Dell EMC PowerEdge C6420 and C6400 Technical Guide   TIA
C6420 w/ Xeon Gold 6248R