I can't connect my Audioengine A2+

I just bought a pair of Audioengine A2+ speakers today and can't connect them to my computer because I had to remove the onboard audio adapter from my motherboard to fit my heatsink (mini itx problems). My headphones work through an Apple USB C to 3.5mm adapter, but the Audioengine can't be found. I read on Google that you're supposed to connect through the default speakers option, but it's disabled because of the adapter being removed. That's why I bought the Apple adapter, but clearly it didn't work. What are my other options?