We do not yet know. Gigabyte specifically says ECC memory is compatible, but run...

ultra2d · on March 1, 2017

I asked Asrock support earlier this week:

  MSI and Gigabyte explicitly mention "(operate in non-ECC mode)". 
  Is that also true for Asrock AM4 motherboards? Or do they fully support ECC?

Reply:

  From: ASRock Support <snip>
  Date: Mon, Feb 27, 2017 at 10:17 AM
  Hello,
  ASRock AM4 motherboards fully support ECC function.

Still, someone will have to test to be sure.