Hi @Vasco Azevedo ,
According to this documentation, When you create a new group, you are not added to the group as a member. After the group is created, add yourself as a member of the group. As I can see you are the owner of the group but not member, Please add yourself as a member too. Then try this API POST https://graph.microsoft.com/v1.0/planner/plans
Hope this helps.
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