I tried most of the previous answers, but it didn’t work for my machine, Ubuntu 18.04 (Bionic Beaver), but what worked for me was this.
First: check your PHP version
$ php -version
Second: add your PHP version to the command. Mine was:
$ sudo apt-get install php7.2-curl
Lastly, restart the Apache server:
sudo service apache2 restart
Although most persons claimed that it not necessary to restart Apache :)