If you’re willing to give up your Wiki service data or if you’ve never even used it in the first place, the solution is easy, just delete the /Library/Server/Wiki folder. For example you can issue the Terminal command:
sudo rm -r /Library/Server/Wiki
and then reboot the computer. You may have to force-kill the Server.app process to be able to reboot.
Run the Server App again to restart the configuration process. You’ll end up with all services intact and Server App will create a new default wiki configuration for you.