The most common cause for this error is a syntax or compilation issue in your code submission.

You can debug your code inside of the sandbox by interacting with the sandbox application after saving your code, then looking for stack traces in the Sandbox Output. You can also put print statements in your code submission and interact with the sandbox application, then look in the Sandbox Output for your print statements. This is explained more in-depth below.

After changing the code in the Code Editor tab, click Save Code. This will update the code running on your sandbox:

Now try running the sandbox web application by using the functionality you patched (e.g. log in, search, etc) to find the error. 

You will see any errors in the "Sandbox Output" section on the bottom of the screen.

Clicking Sandbox Output will show you any print/stdout/stderr output and any stack traces from your code. For tips on getting output through print statements check out this article.

If you are having trouble debugging your code, please go back to the Platform and chat with using the green chat icon in the bottom right of the screen. Our support team can help!