I am trying to integrate my magento v1.9 store with click & drop. But when I make connection it will show "Connection to your marketplace failed. Please check your details and try again".
For Magento 1 we found that being able to set the WS-I compliance solved this by making the Click and Drop system compatible with Magento API service.
Go to -
'System>Configuration>Services>Magento Core API'
and under General Settings
change the WS-I Compliance
from Yes to No.
Now your Magento API will be compatible with Click and Drop. Works for Magento 1.x. No idea how to do it for Magento 2!