Make sure the lib folder is created properly (should be automatically created when composer installs magento/magento2-base) as that's where the css/source/lib/_lib.less file comes from.

Make sure the lib folder is created properly (should be automatically created when composer installs magento/magento2-base) as that's where the css/source/lib/_lib.less file comes from.

You might be able to fix it by getting a new Magento 2 installation via Composer and copying the lib folder to your current installation.