I am not able to export the Tasks to Google Task or / and Calendar. What is wrong?

The problem is most likely caused by Google Calendar’s default update time for external calendars. Google updates them only once a day, which can cause delays in the integration going live. Unsubscribing from Kanban Tool’s calendar in Google and re-subscribing doesn’t help.
The best option is to wait at least a day to see if your tasks start showing in Google Calendar. They usually do.