How to interpret this yum dependency error?
The version of xorgxrdp currently in EPEL was built against RHEL 7.7, specifically against (among other packages) the xorg-x11-server-Xorg package version shipped in RHEL 7.7, but you are running CentOS 7.6.
It is not currently possible for you to install this package, until CentOS 7.7 is released. EPEL does not generally keep older versions of packages, so there is no older version of xorgxrdp to fall back on.
The last word I have seen (from yesterday) is that CentOS 7.7 is expected out "likely some time between 29 August to 6 September 2019." If you want to get early access, you can enable the CentOS CR repo, where the packages will be staged for a week or so prior to release. They should land in CR "early next week". See the comments in /etc/yum.repos.d/CentOS-CR.repo for an explanation of this repo.
How I figured out that the package was built on RHEL 7.7? I saw that the dependency it required had a higher version than any you had available. Based on this I suspected that the package needed a later RHEL version, as I have seen this sort of thing happen before in the weeks between a RHEL release and the corresponding CentOS release.
To confirm it, I looked on my nearest CentOS 7.6 system and saw the version number of xorg-x11-server-Xorg there was the same as that you had installed, and when I looked on a RHEL 7.7 system I saw the version number there was the same as the one it wanted as a dependency.
Wanted to add this as a comment, but if you have the package installed on another machine you can copy it over and rpm install it. If this is a production environment maybe look at setting up your own local mirror that keeps a certain number of package releases, unlike EPEL which only keeps the latest. This is a pretty silly situation and I ran into the same exact problem.