It`s important to follow all the steps that are described in the missions in order to complete the task. You need to change your request
&property=Count(objectId)
In the video attached to this task shows all the steps that have been performed. Also, you may find some useful information for you about this task in the support topic - here.
Please, make sure that API tracking is enabled (you may find it in Manage >> App settings section). If you still have any questions about this task. You can write to us, we are always happy to help.