Trying to re-install Pages app on iMac with Mojave 10.14.6 and keep getting this message. "Pages can’t be installed on “Macintosh HD” because macOS version 10.15 or later is required."

One cannot assume that a failed attempt to install Pages from the Mac App Store will always result in the secondary offer to update the current version for the operating system one is on. Sometimes it works, and other times it does not — based on personal experience, and of the many requests in the Pages for Mac community where it went either way.

Yes obviously it gives you a warning, but it does give you the opportunity to download anyway,

why did you not do that in the first place.

I am running Mojave and do not have Garageband on this mac, but a newer mac has GarageBand

my Accounts page in the App Store shows this clicking on the cloud download symbol will show this

warning.

All you do is click Download and the version suitable for use in Mojave will download,

this should have been the same for Pages.

Open the App Store, click Store in the menubar and then click on View My Account.

This should open a page full of the apps you have 'Purchased' look

for Pages and click the download symbol which should be a cloud with a downward pointing arrow.

Hopefully it should download the version compatible with Mojave.

I found this 1-year old article that should help you: https://apple.stackexchange.com/questions/376919/old-version-macos-sierra-for-pages-keynote-and-numbers-iwork