Installation failed, reverting ./composer.json to its original content.

gistfile1.txt

s3w47m88@spencer-hill-ubuntu:~/Sites/thewaitstaff.com/site$ composer require advanced-custom-fields/advanced-custom-fields-pro:*
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - squizlabs/php_codesniffer 3.0.2 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.0.2 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.0.2 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - Installation request for squizlabs/php_codesniffer (locked at 3.0.2) -> satisfiable by squizlabs/php_codesniffer[3.0.2].

  To enable extensions, verify that they are enabled in those .ini files:
    - /etc/php/7.0/cli/php.ini
    - /etc/php/7.0/cli/conf.d/10-opcache.ini
    - /etc/php/7.0/cli/conf.d/10-pdo.ini
    - /etc/php/7.0/cli/conf.d/20-calendar.ini
    - /etc/php/7.0/cli/conf.d/20-ctype.ini
    - /etc/php/7.0/cli/conf.d/20-exif.ini
    - /etc/php/7.0/cli/conf.d/20-fileinfo.ini
    - /etc/php/7.0/cli/conf.d/20-ftp.ini
    - /etc/php/7.0/cli/conf.d/20-gettext.ini
    - /etc/php/7.0/cli/conf.d/20-iconv.ini
    - /etc/php/7.0/cli/conf.d/20-json.ini
    - /etc/php/7.0/cli/conf.d/20-phar.ini
    - /etc/php/7.0/cli/conf.d/20-posix.ini
    - /etc/php/7.0/cli/conf.d/20-readline.ini
    - /etc/php/7.0/cli/conf.d/20-shmop.ini
    - /etc/php/7.0/cli/conf.d/20-sockets.ini
    - /etc/php/7.0/cli/conf.d/20-sysvmsg.ini
    - /etc/php/7.0/cli/conf.d/20-sysvsem.ini
    - /etc/php/7.0/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.0/cli/conf.d/20-tokenizer.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Installation failed, reverting ./composer.json to its original content.
s3w47m88@spencer-hill-ubuntu:~/Sites/thewaitstaff.com/site$