upgrade OS

Here's a link to download macOS High Sierra from the App Store. For more information, see the Apple support article How to upgrade to macOS High Sierra. And if you need further help, do post back here!

That message usually means there is a partial download of High Sierra on your mac. Look in your Applications folder for Install macOS High Sierra.app, if you find it move it to the Trash, empty the Trash. Connect your mac to your router using a cabled connection rather than using WiFi. Restart your mac, go back to the App Store High Sierra page and start downloading High Sierra. Do not use your mac while it is downloading, and if other devices share the same internet connection don't use them either.

Yes, you can use Disk Utility to partition as you see fit.

But make sure both macOS versions are compatible with your Mac.

Also remember that your data won't share automatically between the two partitions. So if you place a file in the Music folder (for example) on one partition, it won't appear on the other.

NOPE: this is what comes up

This copy of the Install macOS High Sierra application is damaged, and can‘t be used to install macOS.

Is there a way to partition my hard drive to run Sierra on one portion and a newer OS on the other? I have some applications that will not work if I upgrade.