You can always add the “Read more” break for every post that you make, but that’s not what we want.
You have to edit the theme’s code for this to work.
Go to Appreance>Theme Editor
From the right panel look for content.php and change the following
Use this instead
Basically, we checked if the content displayed is for post details or is it any other page.
Note: the <?php tag should not be included in the content.php file since it’s already there.