In the console log just before the exception that says that the file cannot be found is a log message that says what the file path is. The place where your startup script is placed must match that path.
If the component thinks the script should be in a directory like ...scripts/2/startup.js then you are using the latest version of the component and the path to the script uses the cell name as part of the path, in this case the cell name is '2'. You can edit the cell name and change it to 'default' and the script will be found.
Develop games in your browser. Powerful, performant & highly capable.