Click on the Apply button, then click on the Done button when it activates.
Internet/Network Recovery of El Capitan or Later on a Clean Disk If your model supports Internet Recovery (did not come with DVDs and had Lion or later pre-installed) then do this instead of the above: After the installer has downloaded be sure to save a copy in your Downloads folder. If the computer restarts properly then sign into the App Store and re-download High Sierra.
Open Software Update and install the recommended updates. After you complete the Setup Assistant you will be running a fresh installation of Snow Leopard. When the installation has finished the computer will restart into the Setup Assistant.Continue with the OS X installation and follow the directions. After formatting has finished quit Disk Utility.Click on the Options button and select GUID, click OK button, set the format type to MacOS Extended, Journaled. Set the number of partitions to one (1) from the drop down menu. After Disk Utility loads select the out-dented disk entry from the side list (mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility’s right window.
When the menu bar appears select Disk Utility from the Utilities' menu. After the installer loads select your language and click on the Continue button.Release the key when the Apple logo appears and wait for the loading to finish.Immediately upon hearing the chime hold down the C key.Insert Snow Leopard DVD into the optical drive and restart the computer.Press and hold down the power button until the computer shuts down.