There can be several reasons for this error when starting a Spring Boot application. Some possible causes include:

1. Incorrect configuration in the application.properties or application.yml file.
2. Dependency conflicts or missing dependencies in the project.
3. Errors in the code, such as null pointer exceptions or syntax errors.
4. Incorrectly configured database connections or other external resources.

To debug this issue, you can try the following:

1. Enable debug logging by adding the following line to your application.properties or application.yml file: logging.level.root=DEBUG
2. Rerun the application and check the log output for any clues about the source of the problem.
3. Review the code for any potential issues or errors.
4. Check the configuration of any external resources, such as database connections, to ensure they are correct.
5. If the problem persists, you may need to troubleshoot further by using a debugger or by adding additional logging statements to your code.