Do you use any other extensions that may interfere with Music Mode for YouTube? Can you please try to deactivate any other extensions?

Are you sure that you have checked the latest version of the extension that is available (version 0.0.9)? This error was in the version below 0.0.8. In the new version the issue has been fixed. Please check the latest version of the extension and change your review! If the issue with the error exists in version 0.0.9 as well, try to clear your history.