Mojave os link on app store - damaged\

I had exactly this problem with an installer a while ago.

How to get old versions of macOS - Apple Support (CA)

Downloading the installer via this link solved it for me!

Hope it helps

disconnect your Mac from your LAN or Wi-Fi

go to >System preferences>Date&Time

change the date to anywhere between September 24, 2018 through July 21, 2021

run the installer.

Don't connect it back to the network until the install is complete.

when finished reconnect, set the clock for correct day/time/year then run any updates to the OS if prompted.

Choose Applications from the Finder's Go menu, throw the existing installer away, restart the computer, and download it again.

(217223)

Firefox has been known to have issues accessing the link. Use Safari or a Chromium based browser to access the link from the Apple article.

+1 👍