JSON not supported

Hi,
I created a featured product category but having a hard time finding out how to display that slider on the homepage.
Creating a new page or a new post and adding the shortcode but this message appears: \"Publishing failed. The response is not a valid JSON response\"
Therefore, the slider I created saying this \"There has been a critical error on this website.\" when I clicked the view slider.
Please help.
Thank you for your prompt response
Hannah