php artisan command not working in ubuntu

When i do php artisan or php artisan serve in my laravel project it shows some error and I don’t know how to solve it. I tried removing composer and again installing it also doing composer update but it does not seem to solve the problem. Operationg System: Ubuntu 20.04 LTS

My Error:

wiggly@Wiggly-Faun:~/Desktop/Web Designing/websites/Ticketing$ php artisan serve
PHP Warning:  require(/home/wiggly/Desktop/Web 
Designing/websites/Ticketing/vendor/autoload.php): failed to open stream: No such file or 
directory in /home/wiggly/Desktop/Web Designing/websites/Ticketing/artisan on line 18
PHP Fatal error:  require(): Failed opening required '/home/wiggly/Desktop/Web 
Designing/websites/Ticketing/vendor/autoload.php' (include_path='.:/usr/share/php') in 
/home/wiggly/Desktop/Web Designing/websites/Ticketing/artisan on line 18

When I do composer install inside my project is shows:

wiggly@Wiggly-Faun:~/Desktop/Web Designing/websites/Ticketing$ 
composer install
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 v7.18.0 -> satisfiable 
by laravel/framework[v7.18.0].
- laravel/framework v7.18.0 requires ext-mbstring * -> the requested 
PHP extension mbstring is missing from your system.
Problem 2
- Installation request for league/commonmark 1.5.1 -> satisfiable by 
league/commonmark[1.5.1].
- league/commonmark 1.5.1 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 2.0.7 -> satisfiable by 
facade/ignition[2.0.7].
- facade/ignition 2.0.7 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 8.5.8 -> satisfiable by 
phpunit/phpunit[8.5.8].
- phpunit/phpunit 8.5.8 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 v7.18.0 requires ext-mbstring * -> the requested 
PHP extension mbstring is missing from your system.
- facade/flare-client-php 1.3.2 requires illuminate/pipeline 
^5.5|^6.0|^7.0 -> satisfiable by laravel/framework[v7.18.0].
- Installation request for facade/flare-client-php 1.3.2 -> 
satisfiable by facade/flare-client-php[1.3.2].

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-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-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-zip.ini
You can also run `php --ini` inside terminal to see which files are 
used by PHP in CLI mode.

Answer

Did you installed all the necessary vendor packages locally? Does your “vendor” directory exists?

Don’t forget to use:

composer install

in the project directory to install necessary vendor packages.

Also, make sure you have installed necessary PHP extensions, such as mbstring and xml, you may install them via:

sudo apt install php-xml php-mbstring

Hopefullly this helps solve your question.