Add person to person field sharepoint - powershell

Question

Currently I have this script as test:
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName(“System.Web”) | Out-Null
$User = 'usernamehere'
$Pass = ConvertTo-SecureString 'passwordhere' -AsPlainText -Force
$SPOUrl = "sharepointlisthere"
$SPOList = "Add2AD"
$SPListItemColumns = #{
FirstName = "Title"
LastName = "Achternaam"
Title = "Functie"
Managerf = "Teamleider"
Status = "Status"
Mail = "Mail"
Laptop = "Laptop"
Accountnaam = "Accountnaam"
}
#Connect to SharePoint Online
$Context = New-Object Microsoft.SharePoint.Client.ClientContext($SPOUrl)
$Context.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($User, $Pass)
#Get the list
$List = $Context.Web.Lists.GetByTitle($SPOList)
$Query = [Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery(100)
$Items = $List.GetItems($Query)
$Context.Load($Items)
$Context.ExecuteQuery()
foreach($item in $Items)
{
#Get list item for later reference
$ListItem = [Microsoft.SharePoint.Client.ListItem]$listItem = $List.GetItemById($Item.FieldValues["ID"])
if ($item.FieldValues[$SPListItemColumns.Status] -eq "Nee") {
Write-Verbose "Processing list item $Firstname $LastName with ID=$($item.FieldValues["ID"])"
#Put the fieldvalues in variables.
$Managerf = $item.FieldValues[$SPListItemColumns.Managerf]
$Managerf = $Managerf.LookupValue
Write-Output $Managerf
$ListItem[$SPListItemColumns.Accountnaam] = "sgouman#test.nl"
$ListItem.Update()
$Context.ExecuteQuery()
}
}
But whenever I run this script I get this error:
Exception calling "ExecuteQuery" with "0" argument(s): "There are invalid data used to update this listitem or the field is readonly."
so what could it be? I already tried with the user as email address and user as just the normal name none of these work. I want to update the name that is in office 365 so we can later use that field to email them.