Q:

"Unable to create new image" WordPress

This problem looks like it's not had a clear solution as yet. I have installed a clean version of WordPress via SSH with folder permissions set to 755 and file permissions to 644.
I have no problem uploading images, updated plugins etc, but when it comes to editing an image I receive this error:

WP_DEBUG shows nothing, and there are no other errors on the website.

A:

The most likely reason is not having mod-gd installed. 
WordPress uses the GD Graphic Library to manipulate images (see wp-admin/includes/image.php), and this error appears when it is not installed.
It can also be caused by whitespace introduced somewhere. Check for any php closing tags ?> and remove them. They should not be used because whitespace can be introduced after them.

A:

Same problem here, I forgot to install gd with php...
Install it fix the problem :
sudo apt install php7.2-gd

