If your WordPress installation gives an ‘Unable to create image’ error when you try to edit or scale an image in your WordPress media library, it may be because PHP GD isn’t installed. PHP GD is a graphics library used for graphics and drawing, and installing it has resolved the issue for some users.
If you’re using PHP 7.2, then you can install it using:
sudo apt-get install php7.2-gd
If using PHP 7.3:
sudo apt-get install php7.3-gd
Afterwards, restart apache:
sudo service apache2 restart