Because for regular AMI BIOS, two shot beeps relate to the RAM, also from asus two beeps info
https://www.asus.com/support/faq/1029959 that why I ask the RAM info.
Try clear the CMOS, if you don't try it yet.
Also double check the RAM support list, I don't see your RAM ( HX424C15FBK2/8) is in the list, maybe I am wrong. Here is the list https://www.asus.com/us/Motherboards/PRIME-X370-PRO/HelpDesk_QVL/
Add> Sorry, it is in the kingston list kingston memory finder.
http://www.kingston.com/us/memory/search?DeviceType=7&Mfr=ASU&Line=PRIME%20X370%20-%20PRO&Model=96066