The method was deleted in v12 of Veeam. There is a new method that may be used to get same results:
$Repos = Get-VBRObjectStorageRepository -Type AmazonS3Compatible
$Repos | ForEach-Object {
$usedTotal = [Veeam.Backup.Core.CArchiveRepository]::GetTotalUsedSize($_.Id)
$repo | Select Name, Description, @{n='UsedSpace; e={$usedTotal/1TB}}
}