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To resolve the java.lang.NoClassDefFoundError: com/beust/jcommander/ParameterException error when running TestNG using IDEA, follow these steps:

Step 1: Add JCommander dependency to your project

Add the following dependencies to your project's pom.xml file:

<dependency>
    <groupId>com.beust</groupId>
    <artifactId>jcommander</artifactId>
    <version>1.64</version>
</dependency>

If you're not using Maven, you can download the JAR file from this link https://mvnrepository.com/artifact/com.beust/jcommander/1.64 and add it to your project's classpath.

Step 2: Rebuild your project to get the latest changes

After adding the dependency to your project, you need to rebuild your project to update the classpath and make the updated dependencies available.

Step 3: Verify that the classpath is updated

Verify that the JCommander dependency is added to your project's classpath. You can do this in IDEA by going to File -> Project Structure -> Modules -> Dependencies.

Step 4: Clean and rerun your TestNG tests

Clean your project, rebuild it and then rerun your TestNG tests. If you still face the same error, try restarting IDEA and then rerunning the tests. This should resolve the java.lang.NoClassDefFoundError: com/beust/jcommander/ParameterException error.