我已经安装了作曲家，现在正试图运行composer require maknz/slack。然而，我一直在收到错误，我似乎无法修复。我试图安装mbstring，但它仍然给我这个错误？我做错了什么？
- Installation request for maknz/slack ^1.7 -> satisfiable by maknz/slack[1.7.0].
- maknz/slack 1.7.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
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.