Recognize first that you are using a version of SlickEdit that was shipped 10 years before Java 18, so this might be good time to upgrade.
But you can manually create a Java tag file. Tools > Tag Files > New..., then select Java as the language. I don't recall if v16 had the guided dialog for creating a language specific tag file, if not, just create the empty tag file, then click on "Add Files" and add src.jar from your JDK installation directory.