We currently do not build RPMs for CentOS 8, so this is expected behavior at the moment.
However, that will be changing in the very near future. There’s an open PR to overhaul our RPM and DEB package building infrastructure here, which will, among other changes, finally add CentOS 8 RPMs to our repositories.
The current plan is to merge that PR on Monday (2021-03-15), which would translate to having CentOS 8 packages in the edge repo starting from 2021-03-16 and in the main repo starting with the next stable release (tentatively scheduled for 2021-03-24, though that may get pushed back).