The WACS self-hosting plugin (win-acme) starts a webserver on port 80.
For you to use that plugin, you might need to first stop your IIS server, so that WACS can bind to port 80 instead. Then you can start IIS again afterwards. The docs mention:
This plugin launches a temporary built-in web listener that stores the validation response in memory. It can share port 80 with IIS and other (Microsoft) software so this doesn’t interfere with regular traffic. Not all software supports this port sharing feature though. If you get errors telling you that the listener cannot be started, try to (temporarely) shut down other processes using the port, or look for another validation method.
Maybe give that a shot, to see if it's the cause of your problems.
If that is not convenient for you, you should look into using one of the other plugins, like the Filesystem one.