Can you confirm that you’re using Appium Server 1.8.1 (or Appium Desktop 1.6.3)? For maximum compatibility, those are recommended.
Note that you can you Appium Server 1.8.2-beta for iOS 12 and Xcode 10 if necessary, but there’s not corresponding version of Appium Desktop that I’m aware of.
Here’s some more information on troubleshooting some common errors:
If you continue to get errors, could you please share a screenshot and/or log file?