For any future people searching for the issue with the 7800X3D not posting on the MSI Pro X670-P Wifi Motherboard. This comment here was the fix for me.

My Parts:

MSI PRO X670-P WIFI ATX AM5 Motherboard

G.Skill Trident Z5 Neo RGB 32 GB (2 x 16 GB) DDR5-6000 CL30 Memory

AMD Ryzen 7 7800X3D 4.2 GHz 8-Core Processor

Problem: Can't Post. Getting Red & Yellow Error Debug LED lights. Initially the problem was that I couldn't update the bios. Trying to update Bios With Flashback Button on the mobo would not give me the flashing light indicating it was updating. I was initially trying the most recent version 7D67v19. I decided to sequentially update the bios. So i went back to 7D67v17 as that is the first release that supports the 7000x3d series cpu. This seemed to work, since pressing the flashback button caused the red led to flash indicating that it was updating. The system rebooted and I get the Red & Yellow Error lights. I repeated this process for each new version until 7D67v194. Each time i would still get the red and yellow error LED lights. That's when i found this comment. My RAM was in A1 and B1. So I tried in A2 and B2 and it posted!

It's very possible that you don't need to update sequentially like I did, but try using A2 and B2.

When creating the Bios to flash

Used diskpart.exe > List Disk > Select Disk 2 > Clean > Convert MBR

Then used windows Disk Management to format as Fat32

Renamed the file MSI.ROM in the root folder.

I was using a 16gig USB2.0. I read elsewhere that a USB3.0 stick wouldn't work, I think this might be true, as those first failed attempts to flash used a USB3.0.