When using Flask, why does it display the message "Failed to load resource: the server responded with a status of 404 (NOT FOUND)" ?

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This error message typically appears when a requested resource (such as a file or URL) cannot be found on the server. There are several reasons why this may happen when using Flask, including:

1. Incorrect file path or URL: If the requested resource has an incorrect file path or URL, Flask will be unable to find it on the server.

2. Misconfigured server settings: Sometimes server settings may be misconfigured (such as incorrect permissions or improperly configured virtual environments), which can prevent Flask from accessing the requested resource.

3. Missing files: If the requested resource is a file that has been deleted or moved from its original location, Flask will be unable to find it and return a 404 error.

To resolve this issue, you will need to identify the root cause of the error and take appropriate steps to correct it. This can include verifying file paths and URLs, ensuring that server settings are properly configured, and restoring any missing files.