To be able to send an email using a template there must exist an HTML version of the template.
When a template is created via API the HTML version must be provided in the API call (https://api.mailjet.com/v3/REST/template/{template_ID}/detailcontent).

If an MJML template is provided to the API end-point, the API doesn't support creating the needed HTML version of the template that must exist for the template to possible to use.

The only way to get an uploaded MJML template to work is to manually re-publish the API-created template via the web interface. This causes the email editor to automatically create an HTML version of the MJML content.

Please add functionality that automatically generates the needed HTML version of the template, when an MJML template is uploaded via the API.