Drupal needs a PHP library to manipulate images. If it is not installed in your configuration, you must do it because without it your Drupal installation will not work!
You can check with a phpinfo.php page created with your favorite text editor and published on your website :
<?php phpinfo(); ?>
Install the library with apt-get for 7.3 php release
$ sudo apt-get install php7.3-gd
Enjoy!