This seems like LimeSurvey is not recognizing your namespaced classes, likely due to autoloading issues. You need to make sure that the autoloader is picking up PluginCore and TemplatePlugin. Then you need to try including the files if needed.

Furthermore, you need to check if config.xml correctly registers your plugin and follows LimeSurvey’s naming conventions. In other cases, test it with a non-namespaced version to see if the issue is namespace-related. I have practiced these steps for one of the clients website where they were focusing mainly on tech based services.