I think i have found the issue!
When i add a user (by UPN) saved as variable, then the Teams won't let anyone create Planner tabs.
Although the Teams is being created AND the owner is being added... But wont let them create Planner-tabs.
Run as Admin:
$Group = New-Team -DisplayName "ABC Team" -Description "ABC Team" -Accesstype Private
$Teamadmin1 = "firstname.lastname@example.org"
Add-Teamuser -Group.GroupId -user $Teamadmin1 -Role Owner
Can you repeat this and then login as that user who has been assigned as the Owner.
Try to create a Planner-tab.... it will be prevented by an error...
It's far from obvious, but you need to add the user as a member first for this to work, then promote to owner either with PowerShell or in the Teams client.