Enable publishing features
SharePoint Online includes a set of features that enables you to author and publish rich webpages for your intranet or Internet. These features are housed within the SharePoint Publishing Infrastructure and must be activated prior to use. You can activate them at either the site collection or site level.
Activate publishing at the site collection level
From the root of your site collection, click Settings > Site settings.
On the Site Settings page under Site Collection Administration, click Site collection features.
On the Site Features page, scroll down to SharePoint Server Publishing Infrastructure and click Activate.
Once publishing features are activated, subsites inherit them automatically. But, if you built a site before activating the publishing infrastructure, you’ll have to manually activate it for each site.
Activate publishing at the site level
From any page within the site, click (gear icon) Settings > Site settings.
On the Site Settings page, in the Site Actions section, click Manage site features.
On the Site Features page, next to SharePoint Server Publishing, click Activate.