Adding products to cart returns a plain text json response

Hi.
I installed Open cart version 4.0 and all steps was successful. however when I add a product to the cart I get response in plain text json. i have mentioned the response below.

{"success":"Success: You have added <a href=\"https:\/\/stores.justgoonline.online\/index.php?route=product\/product&amp;language=en-gb&amp;product_id=40\">iPhone<\/a> to your <a href=\"https:\/\/stores.justgoonline.online\/index.php?route=checkout\/cart&amp;language=en-gb\">shopping cart<\/a>!"}

Any fixes to this problem ?

Got a link to your store?