If that box is unopened, you should be good to return the X299 Dark for an exchange, maybe they won't hit you with a restocking fee. If this rig is primarily for gaming, your best bet is to stick with the i9 9900K and get a Z390 motherboard, it has to be Z390 to work with the i9 9900K.
Any one of these Z390 boards would work just fine:
Back in the day ya picked motherboards that "worked" having the least bugs with peripheral components.
How do you know which is a better motherboard? I figure a EVGA motherboard matched to a EVGA GPU because all the quirks to a complex system ARE NEVER explained to the consumer. Most likely there are probably some hardware / software enhancement to same brands married to each other. I know they all have NVIDIA chipsets but I'll bet some engineer put a little "extra" for mating EVGA GPU to EVGA CPU. Again went to a couple "best motherboards for 2019" and no detailed analysis. Just testimonials.
I just bought a Pioneer "radio" AVIC-W8500NEX and requires a 64GB USB to be moved back and forth multiple times between the unit and an internet source. To make it more complex the physical hardware is a 3rd party vendor AXXIS , then Pioneer for basic operations and another 3rd party based in Hungary (only email) to get the GPS / mapping unit up to date. Then Sirius XM , Car space harmonic calibration etc. Trying tio figure out how to get up to date traffic, and resoluteion to all the cameras plus have them record to a micro SD card.
For this build ; picking a power supply rationale and priority is length of warranty first , then watts and wiring.
So of these 3 motherboards which work best? With my above comments all be equal Im getting EVGA because of EVGA GPU.