you need to manually add the JUnit JAR files to your classpath since you are not using Maven or Gradle. Download the JUnit JAR files (e.g., junit-4.13.2.jar and hamcrest-core-1.3.jar), place them in a lib/ directory inside your project, and update your .vscode/settings.json file to reference them.

Ensure your test file correctly imports JUnit and try compiling your test class with javac -cp "lib/junit-4.13.2.jar;lib/hamcrest-core-1.3.jar;." MyTest.java, then run it with java -cp "lib/junit-4.13.2.jar;lib/hamcrest-core-1.3.jar;." org.junit.runner.JUnitCore MyTest.

Also, make sure you have the Java Extension Pack installed in VS Code and restart the editor after making these changes.