check this - Githubissues

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 laravel/framework v7.1.3 -> satisfiable by laravel/framework[v7.1.3].
laravel/framework v7.1.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. Problem 2
Installation request for league/commonmark 1.3.1 -> satisfiable by league/commonmark[1.3.1].
league/commonmark 1.3.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. Problem 3
Installation request for tijsverkoyen/css-to-inline-styles 2.2.2 -> satisfiable by tijsverkoyen/css-to-inline-styles[2.2.2].
tijsverkoyen/css-to-inline-styles 2.2.2 requires ext-dom * -> the requested PHP extension dom is missing from your system. Problem 4
Installation request for facade/ignition 2.0.1 -> satisfiable by facade/ignition[2.0.1].
facade/ignition 2.0.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. Problem 5
Installation request for phar-io/manifest 1.0.3 -> satisfiable by phar-io/manifest[1.0.3].
phar-io/manifest 1.0.3 requires ext-dom * -> the requested PHP extension dom is missing from your system. Problem 6
Installation request for phpunit/php-code-coverage 7.0.10 -> satisfiable by phpunit/php-code-coverage[7.0.10].
phpunit/php-code-coverage 7.0.10 requires ext-dom * -> the requested PHP extension dom is missing from your system. Problem 7
Installation request for phpunit/phpunit 8.5.2 -> satisfiable by phpunit/phpunit[8.5.2].
phpunit/phpunit 8.5.2 requires ext-dom * -> the requested PHP extension dom is missing from your system. Problem 8
Installation request for scrivo/highlight.php v9.18.1.1 -> satisfiable by scrivo/highlight.php[v9.18.1.1].
scrivo/highlight.php v9.18.1.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. Problem 9
Installation request for theseer/tokenizer 1.1.3 -> satisfiable by theseer/tokenizer[1.1.3].
theseer/tokenizer 1.1.3 requires ext-dom * -> the requested PHP extension dom is missing from your system. Problem 10
laravel/framework v7.1.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
laravel/ui v2.0.1 requires illuminate/console ^7.0 -> satisfiable by laravel/framework[v7.1.3].
Installation request for laravel/ui v2.0.1 -> satisfiable by laravel/ui[v2.0.1].

To enable extensions, verify that they are enabled in your .ini files:
/etc/php/7.2/cli/php.ini
/etc/php/7.2/cli/conf.d/10-mysqlnd.ini
/etc/php/7.2/cli/conf.d/10-opcache.ini
/etc/php/7.2/cli/conf.d/10-pdo.ini
/etc/php/7.2/cli/conf.d/20-apcu.ini
/etc/php/7.2/cli/conf.d/20-apcu_bc.ini
/etc/php/7.2/cli/conf.d/20-calendar.ini
/etc/php/7.2/cli/conf.d/20-ctype.ini
/etc/php/7.2/cli/conf.d/20-exif.ini
/etc/php/7.2/cli/conf.d/20-fileinfo.ini
/etc/php/7.2/cli/conf.d/20-ftp.ini
/etc/php/7.2/cli/conf.d/20-gd.ini
/etc/php/7.2/cli/conf.d/20-gettext.ini
/etc/php/7.2/cli/conf.d/20-iconv.ini
/etc/php/7.2/cli/conf.d/20-json.ini
/etc/php/7.2/cli/conf.d/20-mysqli.ini
/etc/php/7.2/cli/conf.d/20-pdo_mysql.ini
/etc/php/7.2/cli/conf.d/20-phar.ini
/etc/php/7.2/cli/conf.d/20-posix.ini
/etc/php/7.2/cli/conf.d/20-readline.ini
/etc/php/7.2/cli/conf.d/20-shmop.ini
/etc/php/7.2/cli/conf.d/20-sockets.ini
/etc/php/7.2/cli/conf.d/20-sysvmsg.ini
/etc/php/7.2/cli/conf.d/20-sysvsem.ini
/etc/php/7.2/cli/conf.d/20-sysvshm.ini
/etc/php/7.2/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.

richardkeep / covid-19

check this #2