I cannot add a new attribute. When I put all the information and hit the save button I get an error :
"Class Magento\Framework\Serialize\Serializer\Json does not exist"
How to solve this issue?
check if the Serialize folder exits under vendor/magento/framework. If not try adding the serialize folder.
Seems that some custom module silently depends on the Magento version 2.2+ without a dependency declared it in the
composer.json. Try to search in your project which module is using this class and try to downgrade it to an older version compatible with your Magento version.