Running tests driven by WebDriver, I experienced the following issue.
ERROR: Cannot define class using reflection at new RuntimeError (.\node_modules\webdriverio\build\lib\utils\ErrorHandler.js:144:12)
This occurred after, and proved to be a direct result of, a Java update.
To solve this, sadly you'll have to downgrade from JDK 10 to JDK 9 on your local machine (until Selenium WebDriver is updated to be compatible with Java 10 and until you're ready to update WebDriver on your project).
You can find legacy JDK versions in the Oracle Java Archive.