Issue Cause Solution PayPal button doesn't appear Credentials not configured or mode mismatch Verify Client ID and Secret in PayPal settings. Check Sandbox/Production mode matches credentials. "Invalid Client ID" error Wrong credentials or typo Copy credentials directly from PayPal Developer Dashboard. Check for extra spaces. Sandbox payments work, production fails Sandbox credentials configured in production mode Replace with production credentials. Ensure Sandbox Mode toggle is OFF. IPN notifications not received IPN URL not configured or incorrect Verify IPN URL in PayPal Settings. Test with IPN Simulator in Developer Dashboard. Payment succeeds but invoice doesn't update IPN not processing or permission issue Check WordPress error logs. Verify user permissions for payment updates.