Red LED after changing CPU, doesn't boot

I've just upgraded a CPU in my Mac Pro 2013, from original Xeon 1650v2 to 2690v2. Unfortunately, despite following up to every step with the teardowns/upgrade videos, right now it doesn't boot, or light the white frame around ports.

What it does (when I remove the cover), it lights RED LED immediately when I plug in the power cable.

What's wrong? Where to look for information how to troubleshoot?