I know it's easy to misunderstand, but it's what it is. Even if you are already the owner of the group, but you still need to add you to the members list. See the official doc:
Note: The user who is creating the plan must be a member of the group that will own the plan. When you create a new group by using Create group, you are not added to the group as a member. After the group is created, add yourself as a member by using group post members.
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