To use "var" variables from send API 3.1 you should use this way:

{{var:variableName:""}}

If you need to use data variables it is another syntax:

[[data:contactPropertyName:defaultValue]]

Documentation reference: https://documentation.mailjet.com/hc/en-us/articles/16886347025947-Mailjet-Templating-Language#h_01H57492B40WF1791NNH25XG5X