Add ServiceAccount member to all teams via Powershell

I created a small powershell script to add my ServiceAccount user to all teams via "Add-TeamUser". Unfortunately I get this error message

Add-TeamUser : Error occurred while executing Add-TeamUser
Code: Authorization_RequestDenied
Message: Insufficient privileges to complete the operation.
InnerError:
  RequestId: 25ee1e0c-70cc-44bd-bfe9-b08d5bf05c36
  DateTimeStamp: 2020-07-21T10:19:32
HttpStatusCode: Authorization_RequestDenied
At C:\Users\XYZ\Documents\PSScripts\Others\AddTeamsMember.ps1:5 char:5
+     Add-TeamUser -User ServiceAccount@evgiii.onmicrosoft.com -GroupId ...
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Add-TeamUser], ApiException
    + FullyQualifiedErrorId : Microsoft.TeamsCmdlets.PowerShell.Custom.ErrorHandling.ApiException,Microsoft.TeamsCmdlets.PowerShell.Custom.AddTeamUser

My script looks like this:

Connect-MicrosoftTeams
$groups = Get-Team |Select GroupId, DisplayName

foreach($group in $groups){
    Add-TeamUser -User ServiceAccount@evgiii.onmicrosoft.com -GroupId $group.GroupId -Role Owner
}