I have tried this code but it is not working.

$site = [Microsoft.SharePoint.SPSite]("Your site name")
$web = $site.OpenWeb()
$list = $web.Lists["Your List name"]
$item = $list.GetItemById(Your Id)
AddAttachment $item "Your Document Name"

function AddAttachment($item, $filePath)
{
$bytes = [System.IO.File]::ReadAllBytes($filePath)
$item.Attachments.Add([System.IO.Path]::GetFileName($filePath),$bytes)
$item.Update()
}

I have tried this code to attach a document in custom list for sharepoint online, but it is not working.

$site = [Microsoft.SharePoint.SPSite]("Your site name") $web = $site.OpenWeb() $list = $web.Lists["Your List name"] $item = $list.GetItemById(Your Id) AddAttachment $item "Your Document Name" function AddAttachment($item, $filePath) { $bytes = [System.IO.File]::ReadAllBytes($filePath) $item.Attachments.Add([System.IO.Path]::GetFileName($filePath),$bytes) $item.Update() }