This is weird. I have used many ASUS MBs specified as "can run ECC and non-ECC" and this has always meant that there was an ECC enabling option in the BIOS settings, and then if the OS had an appropriate EDAC driver for the installed CPU ECC worked fine.

I am writing this message on such an ASUS MB with a Ryzen CPU and working ECC memory. You must check that you actually have a recent enough OS to know your Threadripper CPU and that you have installed any software package required for this (e.g. on Linux "edac-utils" or something with a similar name).