When did you purchase the Asus Motherboard, before 05/01/2019?
If you just recently purchased the Asus Motherboard it should have BIOS version 2301 or newer on it unless the Motherboard was manufactured before 05/2019.
You need BIOS version 2301, which came out on 05/15/2019, for the Motherboard to recognize and support the Ryzen 9 3900X Processor.
Go to your Motherboard and look for a small white thin label near the Motherboard CPU Socket. I believe on the second line at the end will have a 4 digit number. That is the BIOS version installed on your motherboard.
If the 4 digit number is 2301 or higher that means your motherboard will support your processor out of the box.
If the number is lower than 2301 then you purchased a rather old motherboard in which case I would check with Asus Support to see when the Warranty will end.
If the number is 2301 or higher than you have another issue for the motherboard not working with your processor.
