My preferred way of creating a new site template from an existing site or subsite is just appending _layouts/15/savetmpl.aspx to the end of the site’s URL. However if saving sites as templates isn’t enabled for the site from which you’re creating a template, you’ll get the following error:
The “Save site as template” action is not supported on this site.
This error is just referring to a site property known as SaveSiteAsTemplateEnabled currently set to false.
This can be remedied (property changed to true) with a little bit of PowerShell ran on a SharePoint server. Run PowerShell as an administrator then run the following script, replacing the site URL with your own site or subsite’s URL.
Add-PSSnapin "Microsoft.SharePoint.PowerShell"
$web = Get-SPWeb https://WebApp/sites/SubsiteIfApplicable
$web.AllProperties["SaveSiteAsTemplateEnabled"] = "true"
$web.Update()Once it completes, attempt to save the site as template again. You should now be able to proceed with saving the SharePoint site as template, and see the fields shown below:
Note that if a user activates the site collection feature SharePoint Server Publishing Infrastructure or the site feature SharePoint Server Publishing, you’ll need to run the PowerShell command again because activating those features includes disabling the SaveSiteAsTemplateEnabled property.