laravel/framework requires ext-mbstring

I have the same problem when installing Laravel 6 and getting an error like this

Installing laravel/laravel (v6.12.0)
  - Installing laravel/laravel (v6.12.0): Loading from cache
Created project in sistemLogin
> @php -r "file_exists('.env') || copy('.env.example', '.env');"
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
    - laravel/framework v6.9.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.8.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.7.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.6.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.6.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.6.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.5.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.5.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.5.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.4.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.4.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.3.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.2.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.17.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.17.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.16.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.15.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.15.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.14.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.13.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.13.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.12.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.11.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.10.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v6.10.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework 6.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for laravel/framework ^6.2 -> satisfiable by laravel/framework[6.x-dev, v6.10.0, v6.10.1, v6.11.0, v6.12.0, v6.13.0, v6.13.1, v6.14.0, v6.15.0, v6.15.1, v6.16.0, v6.17.0, v6.17.1, v6.2.0, v6.3.0, v6.4.0, v6.4.1, v6.5.0, v6.5.1, v6.5.2, v6.6.0, v6.6.1, v6.6.2, v6.7.0, v6.8.0, v6.9.0].

  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-mysqlnd.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-calendar.ini
    - /etc/php/7.4/cli/conf.d/20-ctype.ini
    - /etc/php/7.4/cli/conf.d/20-curl.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-json.ini
    - /etc/php/7.4/cli/conf.d/20-mysqli.ini
    - /etc/php/7.4/cli/conf.d/20-pdo_mysql.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-zip.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

And I try command:

sudo apt-get install php-mbstring 

After that I tried again install laravel and there were still problems like this

    Installing laravel/laravel (v6.12.0)
  - Installing laravel/laravel (v6.12.0): Loading from cache
Created project in sistemLogin
> @php -r "file_exists('.env') || copy('.env.example', '.env');"
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
    - phpunit/phpunit 8.5.x-dev requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 8.5.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 8.5.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 8.5.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    ...........

So if like that, you can try command

sudo apt-get update
sudo apt install php-xml

then run

composer update
composer require cviebrock/eloquent-sluggable

PHP7 : install ext-dom issue


Install the required mbstring extension

sudo apt-get install php-mbstring 

In Dockerfile you should add:

RUN docker-php-ext-install mbstring

sudo apt install php-mbstring will work and will download the plugin for the latest PHP version.

If your PHP version is not the latest, then you can add version in it:

# PHP 7.1
sudo apt install php7.1-mbstring

# PHP 7.2:
sudo apt install php7.2-mbstring

# PHP 7.3
sudo apt install php7.3-mbstring

# PHP7.4
sudo apt install php7.3-mbstring

# PHP8
sudo apt install php8.0-mbstring

# PHP8.1
sudo apt install php-mbstring