The ROG Maximus XI Hero Black Ops 4 edition supports 8th and 9th generation cpu's. You'll likely have to update to the latest bios for the board to recognize the i9-9900k. Updating the bios is easily done with the bios flashback feature.
All you need is power to the board and a usb flash drive with the latest bios which you can download from the ASUS Support Center.