Your requirements could not be resolved to an installable set of packages. - Symfony Project
This error means that there is some package incompatibility in your composer.json file. If you want to force, run composer install --ignore-platform-reqs
But I suggest you to check wich dependencies version are not compatible with each other
I think your problem is caused by combination of these two requirements:
"symfony/symfony": "3.4.*""symfony/css-selector": "^4.2"Symfony/symfony package has symfony/css-selector in it's replace section with self.version That means when composer tries to instal symfony/symfony version 3.4.31 it also counts as symfony/css-selector version 3.4.31 which doesn't match the version ^4.2 in your require-dev.
You might also encouter same problem with symfony/browser-kit.
You should revise your requirements to not include multiple versions of same package.