Get-AzureADUserManager : Cannot bind argument to parameter 'ObjectId' because it is null.

Shashikant_Gagare 

It looks like you are trying to get the manager's manager by passing the ObjectId of the $manager object, but the $manager object itself is null. This is causing the Get-AzureADUserManager cmdlet to fail.

To avoid this error, you should first check if the $manager object is not null before trying to get its manager. You can do this by adding a simple if statement:

foreach($user in $users)
{
    $row = $Datatable.NewRow()

    $manager=Get-AzureADUserManager -ObjectId $user.ObjectId

    if($manager -ne $null)
    {
        $seniorM=Get-AzureADUserManager -ObjectId $manager.ObjectId
        $row.seniorM=$seniorM.DisplayName
    }

    $row.Name=$user.GivenName
    $row.Surname=$user.Surname
    $row.manager=$manager.DisplayName

    $Datatable.Rows.Add($row)
}

This way, if the $manager object is null, the if statement will prevent the $seniorM object from being created and the script will continue without throwing an error.