If I'm receiving a 401 error when using an API client, what should I check first?

First, verify your API credentials (e.g., API key, access token) are correct and haven't expired. Check that the credentials are being sent in the correct header (e.g., 'Authorization'). Also, confirm that you have the necessary permissions to access the specific endpoint or resource you're requesting. Review the API documentation to identify any authentication-related nuances.