If your download arrived as a disk image ( .dmg ) file instead of a direct App bundle, you must register it to your system first:

Select your internal hard drive (usually named Macintosh HD) from the left sidebar. Click the button at the top. Set the format type:

His trusty 2011 MacBook Pro, a silver beast with a glowing apple and a heartbeat of spinning platters, was tired. It groaned under the weight of modern web bloat. Elias knew what he needed: a clean slate. A fresh macOS High Sierra download