Hi @suyashcyber94 ,
if you are not able to find the config/lighthouse.php file that means installation is not performed properly.

after running the installation commnad :

php artisan bagisto_graphql:install

you can see in the below link ,this will hit to to following file and you can see the publish code for the lighthouse.php file.

so please make sure the installation is properly perfomed.

Also the config folder is on the root of your project.