Usually this error occurs when WordPress cannot find the graphic library which should be installed on your server.
You need contact the hosting provider or if you hosting your website by using your own server that using XAMPP. then you can solve it very simply.
Open XAMPP control panel, then select Config -> PHP (php.ini)
Search the line “extension=gd”. Now just remove ; to enable gd services at your system for PHP as by default it comes with commented and save the file and restart both services Apache and MySQL.
Then you need restart Apache.