Just use the aorus pro itx. No need to waste time with older b450 boards. The pro itx doesn't use doublers. It's pure 6+2 phase direct. It will work great with 3700x.

Exact same board in my ITX rig. Same problem encountered too. It was my RAM. The board's stock BIOS rev. didn't want to play nice with my Klevv Bolt XR 3600c18 kit.

You have 2 options below;

1. Go find another ram stick, preferably something simple like a 2400/2666 generic ram etc, or just try anything lah. I had a Samsung 2666 8gb stick. That worked. Boot, enter BIOS, flash newer BIOS, use back the intended RAM, no issue already.

2. Or direct get a newer BIOS put it on a USB 2.0 stick and use the BIOS flashback.

The stock BIOS revision on that B550i Aorus Pro AX is very fussy with RAM and does not want to POST with certain ram. I sent the board back to the Shopee seller and they tested fine, wasted time only. Once I was convinced the board itself was working, I figured it out.