If you are using the correct API Key and Secret Key, the issue could be that the sender address being used is not validated on this API Key. Please go the Sender Domains & Addresses page to confirm your sender address status.
In the event the sender address is validated and you continue to receive an error, please contact our Support Team with the following information:
1. The PrestaShop version
2. The Mailjet plugin version
3. Outline the steps you followed to install the plugin
4. A screenshot of the error message
1. The PrestaShop version
2. The Mailjet plugin version
3. Outline the steps you followed to install the plugin
4. A screenshot of the error message