Requirements could not be resolved

I am on Linux Ubuntu and I have a Symfony 4 project and I did not succeed to make that functions. After updating my OS, I have a problem. When I launch for this project:

composer install

I got this message:

Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for nexylan/slack v2.0.0 -> satisfiable by nexylan/slack[v2.0.0].
    - nexylan/slack v2.0.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
  Problem 2
    - Installation request for symfony/framework-bundle v4.0.14 -> satisfiable by symfony/framework-bundle[v4.0.14].
    - symfony/framework-bundle v4.0.14 requires ext-xml * -> the requested PHP extension xml is missing from your system.
  Problem 3
    - Installation request for symfony/debug-bundle v4.0.4 -> satisfiable by symfony/debug-bundle[v4.0.4].
    - symfony/debug-bundle v4.0.4 requires ext-xml * -> the requested PHP extension xml is missing from your system.
  Problem 4
    - symfony/framework-bundle v4.0.14 requires ext-xml * -> the requested PHP extension xml is missing from your system.
    - symfony/maker-bundle v1.4.0 requires symfony/framework-bundle ^3.4|^4.0 -> satisfiable by symfony/framework-bundle[v4.0.14].
    - Installation request for symfony/maker-bundle v1.4.0 -> satisfiable by symfony/maker-bundle[v1.4.0].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php/7.4/cli/php.ini
    - /etc/php/7.4/cli/conf.d/10-opcache.ini
    - /etc/php/7.4/cli/conf.d/10-pdo.ini
    - /etc/php/7.4/cli/conf.d/20-apcu.ini
    - /etc/php/7.4/cli/conf.d/20-calendar.ini
    - /etc/php/7.4/cli/conf.d/20-ctype.ini
    - /etc/php/7.4/cli/conf.d/20-exif.ini
    - /etc/php/7.4/cli/conf.d/20-ffi.ini
    - /etc/php/7.4/cli/conf.d/20-fileinfo.ini
    - /etc/php/7.4/cli/conf.d/20-ftp.ini
    - /etc/php/7.4/cli/conf.d/20-gettext.ini
    - /etc/php/7.4/cli/conf.d/20-iconv.ini
    - /etc/php/7.4/cli/conf.d/20-imagick.ini
    - /etc/php/7.4/cli/conf.d/20-json.ini
    - /etc/php/7.4/cli/conf.d/20-phar.ini
    - /etc/php/7.4/cli/conf.d/20-posix.ini
    - /etc/php/7.4/cli/conf.d/20-readline.ini
    - /etc/php/7.4/cli/conf.d/20-shmop.ini
    - /etc/php/7.4/cli/conf.d/20-sockets.ini
    - /etc/php/7.4/cli/conf.d/20-sysvmsg.ini
    - /etc/php/7.4/cli/conf.d/20-sysvsem.ini
    - /etc/php/7.4/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.4/cli/conf.d/20-tokenizer.ini
    - /etc/php/7.4/cli/conf.d/20-xdebug.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Can you help me to solve this problem?