Save Site As Template " option is not available in sharepoint online

Open site in SharePoint Designer
Go to option attribute save as template =false make it as true

How to enable Save as template site in Site action Categories Site collection level using Powershell

$web = Get-SPWeb "http://spserver/sites/Raghu/"
$web.AllProperties["SaveSiteAsTemplateEnabled"] = "true"
$web.Update()