Hi @Anonymous ,
Can you check if your added as a member to the group, sometimes users who are owners get this error add the user as member also even though the user is already added as owner.
Please refer to teams-owner-cannot-create-planner-tab.
POST https://graph.microsoft.com/v1.0/planner/plans
Content-type: application/json
{
"owner": "{group-guid}",
"title": "{title-value}"
}
Also, refer to this documentation with common planner 403 errors.
Hope this helps.
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