In some countries like India, laptops are sold without Windows OS to save money. You can purchase Windows 10 separately and install it on the laptop; it comes on a bootable USB drive. However, the generic version of Windows 10 that Microsoft sells may not have all the necessary drivers for your specific laptop. That means you may not even be able to connect to the internet because Windows 10 may not have a compatible driver for the ethernet and/or WiFi chip in your laptop.
That means you need to download drivers on to a USB drive / burn to DVD from a different PC / laptop from the manufacturer's website. The drivers must be specific to the laptop model you have.