This plugin will add a new meta box to your posts and pages allowing you to mark posts that require users to login. Visitors trying to view the post, will be asked to login. 
<a href="https://i.imgur.com/ghvTt.png" rel="nofollow ugc">See a screenshot</a>

Simple Content Restriction

Ask your visitors to login if the post is marked as visible only to registered users.

Denis Yanchevskiy

Allows you to make post, page, custom post elements required: title, content, featured image.
After installing the plugin, you need to go to the Settings -> DCO Post Validator to specify the required elements.

DCO Post Validator

Allows you to make post, page, custom post elements required: title, content, featured image.

Simple Content Restriction vs DCO Post Validator