List download as Template not an option SharePoint Online
I have noticed that if you like to download a SharePoint list or library as a template this option is not availiable. After seaching for a sollution i found out that you need to make a adjustment to the site settings. The quickist way is with Powershell. This is a per Site setting.
Set-SPOsite-DenyAddAndCustomizePages 0
You can also change the setting for the whole tenant. More information from Microsoft