You can insert it here.
You can insert it here.
Your PC is starting up fine and the build is nearly complete, but the RGB lights aren’t turning on. It seems the unplugged cables might be the issue. You’re holding one of the three, but you’re unsure how to connect them properly. I’ve read the manual, but it’s still unclear. Make sure all cables are securely plugged in and try a different port if possible.
I believe it's meant for the RGB fans. I doubt your motherboard includes a port for that, even though it does have addressable RGB. The board model is B450 Steel.
I realize now that I missed connecting one of the SATA power cords that actually supplied the RGB. Those three-pin connectors are meant for additional RGB gear or an ARGB hub.