I recently updated my CraftCMS 3 with the help of Composer.
When I try to install a plugin with the plugin store, I get an error saying that Craft couldn't find my composer.json file. He proposes me to define the CRAFT_COMPOSER_PATH in index.php. But, when I do, I still get the same error.
My composer.json is the root of craft folder. I seriously don't know how to solve my problem.
Here's a screenshot (sorry I use Craft in french):