Incompatible types. Found: 'java.lang.Class<junittest.Calculator>', required: 'java.lang.Class<? extends org.junit.runner.Runner>'

This error message typically occurs when trying to run a JUnit test using a version of JUnit that is not compatible with the code being tested.

To resolve this issue, ensure that you are using the correct version of JUnit for your code. If you are using an older version of JUnit, try upgrading to a newer version to see if that resolves the issue. Additionally, make sure that your test class is properly annotated with the `@Test` annotation and that you have imported the necessary JUnit classes.