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This error message typically indicates that the client is attempting to use an outdated or unsupported version of the HTTP protocol to communicate with the server. The server is responding with a "426 Upgrade Required" error code, which indicates that the client needs to upgrade to a newer version of the HTTP protocol in order for the server to process its requests. This error can occur in the context of a React and Node application if the server is expecting a newer version of the HTTP protocol than what the client is using. To resolve this issue, the client may need to update its HTTP library or framework to a version that is compatible with the server's protocol requirements.