Class 'LanguageServer\StderrLogger' not found

[Alexey T.]

I cloned this repo.
I run the main php file with PHP8.0 (installed from XAMPP package to /opt/lampp) or via PHP7.4 (installed from APT) on Ubuntu 20 x64.
Same error--

user@PC:~$ cd ~/repos/php-language-server/bin/
user@PC:~/repos/php-language-server/bin$ php php-language-server.php 
PHP Fatal error:  Uncaught Error: Class 'LanguageServer\StderrLogger' not found in /home/user/repos/php-language-server/bin/php-language-server.php:18
Stack trace:
#0 {main}
  thrown in /home/user/repos/php-language-server/bin/php-language-server.php on line 18

[Damjan Cvetko]

Were you able to resolve the problem?

[Alexey T.]

I did not run Composer.