Upon startup, press ***[command]+[option]+[R]***, this will allow you to install the latest "available+compatible" version of MacOS for your laptop.
For example, on a Macbook 2012, the factory OS shipped with the Macbook is ***MacOS Lion*** version, if you press [command]+[R], it will attempt to re-download the "***Lion***" version, which is not available and it will fail to download. In stead, with [command]+[option]+[R], it will download ***macOS High Sierra***.