Creating a KanBan Planner for an existing project requires the rerun of the workflow to enable the picker.
The steps to remove default planner:
1) Select the project
2) Click on "Workflows"
3) Click on "Remove Default Planner"
PS: Also, the recording shows the same steps in motion.
Once the workflow is done, users are able to select the Kanban board along with the Online Planner.
Credits: Marko Manchev Mar 13, 2018