The error that you are receiving while testing the JSON connection from the Web Design Studio is due to the limitation of the Denodo Platform Express version. The sources with the Denodo Browser client cannot be used to retrieve data from CSV, JSON or XML files in the Denodo Express version.
To know the limitation imposed on the Denodo Express, refer to the [Restrictions of Denodo Express](https://community.denodo.com/docs/html/browse/8.0/en/platform/installation/appendix/limitations_of_the_denodo_express_license/limitations_of_the_denodo_express_license#restrictions-of-denodo-express) section of the Denodo Platform Installation Guide.
If you would like to get a full featured trial license for Denodo, which would allow you to perform this operation in Denodo, please contact our Sales Team at email@example.com or fill out the contact form at https://www.denodo.com/action/contact-us/.
Hope this helps!