You will need macOS Mojave 10.14.* or Catalina 10.15.* installed on the Mac before you can download any {Pages, Numbers, Keynote} v10 from the Mac App Store. There are no updates to the iWork '09 applications as they were retired in early Fall 2013.
Press the option key while selecting the Purchases tab on the Mac App Store on that High Sierra Mac. If you see Pages, Numbers, and Keynote with Update/Install buttons, it will attempt the first paragraph, tell you that you can't, and then it may offer to update these applications to the latest available for High Sierra. Pages v8.1, Numbers v6.1, and Keynote v9.1.
The second paragraph is your only hope of upgrading these applications to the last versions compatible with High Sierra.