While Apple has released several iterations since 10.13, High Sierra remains relevant for several key reasons:
Press the Power button while holding down the key immediately. Release the key when you see the Startup Manager screen. macos high sierra 10136 dmg
If you are unable to download the installer, you may need to restart your Mac into Internet Recovery by holding Shift+Option+Command+R at startup to reinstall the original OS that came with your computer. To help you better, could you tell me: What of Mac are you using? While Apple has released several iterations since 10