You will need the Limited Write Group permission or be an admin in the appropriate group to perform these actions.
When scheduling volunteers on a service plan, the available positions for each team comes from the Positions tab of the group. If a particular team does not have a position you need to schedule, the fastest way to add it is to first click on the group name at the top. This will take you directly to that group.
Alternatively, you can go to Groups in the side menu and locate the group that way.
Then, visit the Positions tab from where you can add new positions and edit the existing ones.
Only active positions appear when scheduling volunteers on a team in a service plan.