You've soldered the microcontroller onto your PCB, but the keyboard doesn't power on. You've tested the microcontroller and verified that it works, but when you plug the keyboard in, nothing happens. What now?
Test each side individually
Don't connect both halves with a TRRS cable at first, and connect a half directly to your computer with a USB cable. Do they work individually, but not when connected together? Then you should read Only one half of my keyboard works at a time, but not when they are both connected.
Does a single half not work? Then you likely have a short, which is what happens when two pins or pads on the PCB are connected when they should not be. You should check the pins on your microcontroller to see if any adjacent pins are connected, and remove the extra solder if they are.
Also check the alignment of your RGB LEDs if you have those: wrongly aligned LEDs can cause shorts too. If you find that the triangle on the LED doesn't match up with the circle on the PCB, you'll need to remove the LED and fix the alignment.
Sometimes, the alignment of the microcontroller isn't right. If you've used a Pro Micro, Elite C or similar, read the step in the build guide to verify your microcontroller is facing the right side. On the Kyria, the microcontrollers should face with their backside upwards, so you shouldn't be able to see the components on the microcontroller.
Test both halves at once
Next up, connect both halves with a TRRS cable, and then connect one of those halves to the computer with a USB cable. The keyboard should now work as you'd expect (though if you're here, it likely won't, yet).
If it doesn't work, unplug the keyboard from your computer, and unplug the TRRS cable. Look at the rings on the TRRS cable and count: does it have four contacts, separated by colored or black rings? Chances are your cable is actually a TRS cable, in which case it only has three contacts.
If you happen to have a second TRRS cable, retry the test with that, sometimes the cable might be broken.
Next up, you'll want to make sure the solder joints on your TRRS jacks and microcontroller are good. Adafruit has an excellent overview of common soldering problems. Compare your solder joints with those in the guide, and if any have problems, rework them to match with the correct examples in the guide.