Name, Department, #of Emails, Mailbox Size

lpmdvip Good afternoon. No, it should not prompt for creds, which means that the cred files is not been read correctly(Please verify that you have the cred file saved to the credentials folder). I've updated the script to use your documents folder as working dir and creds dir. Also, try to run the ISE as admin to avoid permissions issues to write/read the CSV files. Could you try to see what is saved to the variable $Results? If the data is already there, you don't have to run the whole script again just use the line 105 to export the report to a folder location that you have permissions to write on. 

 <#
.SYNOPSIS
    Get MBXs Stats, User Attribute(s), Folder(s) Counts
.DESCRIPTION
    
.NOTES
    Author:     Erick A. Moreno
    Email:      emoreno@request-script.com
    Date:       13 Jan 2020
    PowerShell: v3
.References
#requested: https://techcommunity.microsoft.com/t5/windows-powershell/name-department-of-emails-mailbox-size/m-p/1167911
#>

$CredentialsFolder = "$home\Documents\"
$WorkingDir        = "$home\Documents"
$ScriptTime        = Get-Date -UFormat "%m%d%Y-%H%M"

#region Functions
Function _Get-Sessions
 {
  Param(
        [Parameter(Mandatory=$true)]$CredentialsFolder,
        [Parameter(Mandatory=$true)]$CredentialsFileName
       )
  
  #region Check for Credentials File
  $TargetCredPath = $CredentialsFolder #Credential XML File Location
  $TargetCredName = $TargetCredPath + $CredentialsFileName
  #Write-Host $TargetCredName
  If(Test-Path $TargetCredName)
   {
    $TargetCred = Import-Clixml $TargetCredName
   }
  Else
   {
    $Cred = Get-Credential 
    $Cred | Export-CliXml -Path $(Join-Path -path $TargetCredPath -ChildPath RemotePS.cred)
    $TargetCred = Import-Clixml $TargetCredName
   }
  #endregion
  #region connections
  $Session  = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $TargetCred -Authentication Basic -AllowRedirection 
  $NoOutput = Import-PSSession $Session -AllowClobber -Prefix O365 -CommandName "*Mailbox*","*User*"
  #endregion
  Return  $Session
 }
#endregion
#region Open PS Sessions
$TenantSession = _Get-Sessions -CredentialsFolder $CredentialsFolder -CredentialsFileName RemotePS.cred
#endregion

$MBXs = Get-O365Mailbox -ResultSize Unlimited 

$Counter = $MBXs.count
$Results = @()
$NoDepartment = @()
$Counter

Foreach($MBX in $MBXs)
 {
  #Start-Sleep -Milliseconds 300
  $PsSessionState = $(Get-PsSession $TenantSession.Id).State  
  If($($Counter/100) -match '^[0-9]+$' -or $PsSessionState -ne "Opened")
   {
    Remove-PSSession $TenantSession.Id -Confirm:$false
    Write-Host "Opening a new Session Each 100 iterations" -ForegroundColor Yellow
    $TenantSession = _Get-Sessions -CredentialsFolder $CredentialsFolder -CredentialsFileName RemotePS.cred
   }

  Write-Host "($Counter) Working On MBX: $($MBX.PrimarySmtpAddress)" -ForegroundColor Cyan
  $Stats       = $null
  $UserData    = $null
  $FolderItems = $null
  $UserData    = Get-O365User $MBX.PrimarySmtpAddress

  If(![string]::IsNullOrEmpty($($UserData.Department)))
   {
    $Stats       = Get-O365MailboxStatistics -Identity $MBX.PrimarySmtpAddress
    $FolderItems = Get-O365MailboxFolderStatistics -Identity $MBX.PrimarySmtpAddress | Where {$_.Name -match “Inbox|Sent Items|Deleted Items|Junk E-Mail”} | Select Name,ItemsInFolder #Add Folders as needed
    
    $Properties  = [Ordered]@{
                              DisplayName        = $MBX.DisplayName
                              Department         = $UserData.Department #Add properties as needed
                              UserPrincipalName  = $MBX.UserPrincipalName
                              MailboxSizeMB      = [math]::Round($($Stats.TotalItemSize.Value.ToString().Split("(")[1].Split(" ")[0].Replace(",","")/1MB),4)
                              MailboxSizeGB      = [math]::Round($($Stats.TotalItemSize.Value.ToString().Split("(")[1].Split(" ")[0].Replace(",","")/1GB),4)
                              InboxItems         = $($FolderItems | where{$_.Name -eq "Inbox"}).ItemsInFolder
                              SentItems          = $($FolderItems | where{$_.Name -eq "Sent Items"}).ItemsInFolder
                              DeletedItems       = $($FolderItems | where{$_.Name -eq "Deleted Items"}).ItemsInFolder
                              JunkItems          = $($FolderItems | where{$_.Name -eq "Junk E-Mail"}).ItemsInFolder
                              PrimarySMTPAddress = $MBX.PrimarySmtpAddress
                             }
    
    $Results += New-Object PSObject -Property $Properties
   }
  Else
   {
    Write-Host "Department Field is Empty Skipping MBX: $($MBX.PrimarySmtpAddress)"
    $NoDepartment += $($MBX.PrimarySmtpAddress)
   }
  $Counter--
 }

 $Results | Export-Csv -Path "$WorkingDir\ReportMBXsStats_$ScriptTime.csv" -NoTypeInformation -Encoding UTF8 -Force
 $NoDepartment | Export-Csv -Path "$WorkingDir\NoDepartmentField_$ScriptTime.csv" -NoTypeInformation -Encoding UTF8 -Force
 Write-Host "Report Saved To: $WorkingDir\ReportMBXsStats_$ScriptTime.csv" -ForegroundColor Cyan
 Write-Host "Report Saved To: $WorkingDir\NoDepartmentField_$ScriptTime.csv" -ForegroundColor Cyan