Sometimes those settings aren't set to properly so prestashop won't authorize a 3rd party to connect to their servers.
Go to your Prestashop into "advanced settings" -> "Webservice" and make sure both options are set on "Yes".
The CGI option isn't mandatory if your server isn't compatible.
After creating a new API key in prestashop, you need to make sure it has all the permissions so that ChannelDock can access everything.
In order to do that, go to "advanced settings" -> "webservice" and select the API key you used in ChannelDock. Then tick all the boxes in the permissions and press save.
Sometimes Prestashop isn't set up to allow web services to connect to the API. To fix that you need to make sure that your HTaccess contains this instruction :
SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1