cannot install pages, numbers and keynote

Are you referring to the iWork '09 application suite or newer versions of Pages?

I never lost any Pages, Numbers, or Keynote applications during any upgrade to macOS High Sierra, or any of its updates through 10.13.6 and security updates. You can restore these applications from your Time Machine backup prior to the time of their loss.

Or, launch the Mac App Store, sign in with your Apple ID (if not already). Then press and hold the option key while selecting the Purchase tab. Pages, Numbers, and Keynote should appear here with an Update button. When you click these, you will be notified that you need Catalina or Big Sur to install the current Mac App Store versions (11.1). At that point, it might decide to allow you to reinstall the last versions of these applications for High Sierra — or it may not. Apple does not provide any other means for user downloadable full installers for past versions of these applications.

If you cannot restore from /Applications/Time Machine, and the second paragraph didn't work, then your recourse is to use Pages for iCloud, or Pages for iOS/iPadOS.