Can't download Mojave, the copy of the install Mojave application is damaged and can't be used.

Ok that error message is really saying the App Store has found a copy of the Install macOS Mojave.app (or partial copy)

in your Applications folder.

Open your Applications folder locate Install macOS Mojave.app and move it to your Trash, empty the Trash.

connect your mac to your router via cable rather than rely on WiFi. Restart your mac go back to the Mojave download page in the App Store, it should now say 'Get'. Do not use the mac while it is downloading, do not use other devices that share the same internet connection while the mac is downloading.

In the first post there is no mention of using an old installer, the question is about getting an error message from the App Store while trying to download Mojave. That error message typically means that a copy of Install macOS Mojave.app has been detected in Applications folder, as I wrote.

Why the mention of old installers and expired certificates came up I do not know.

When the Certificates for several of Apples OS's expired in October 2019, they were replaced with

revised versions with new certificates.

Yes it is true that you can install an OS with an expired certificate by changing the date, but I don't think

the issue here is an old installer with an expired certificate.

Below are screenshots of the Install macOS Mojave.app, one downloaded before October 2019

and one downloaded after October 2019, please note the difference in version number.

There’ve been certificate issues with some of the Apple installers, and if this is a freshly-downloaded copy, try setting your system time back to the time when Mojave was released.... Switch to manual system time in System Preferences > Time & Date, and manually set your system time to 30-Sep-2018, launch the installer, and—once you launch the installer and get to the first installer prompt—pause the installer briefly by going into the Apple menu, and set your system time back to automatic. Bring the installer back to the front, and continue.

It's downloading from that link on my computer running Catalina. No error message so far. Have you successfully downloaded the installer? Are you running the installer from a startup volume running Catalina or do you already have a Mojave installer in your Applications folder or elsewhere on the disk?

Eau Rouge wrote:

Ok that error message is really saying the App Store has found a copy of the Install macOS Mojave.app (or partial copy)

in your Applications folder...

The old installer has an old cert, and—particularly on a slow network or a cap’d network—using the system date to work around a re-download does work, if it’s the certificate issue. I ended up using this sequence to load Mojave a month or so back, having encountered this same error. FWIW.

Eau Rouge wrote:

In the first post there is no mention of using an old installer, the question is about getting an error message from the App Store while trying to download Mojave. That error message typically means that a copy of Install macOS Mojave.app has been detected in Applications folder, as I wrote.

The damaged-and-can’t-be-used is a failed install attempt. I hit this exact combination with a Mojave installer, too.

The copy of the install macOS Mojave application is damaged and can't be used to install macOS.

The OP was clearly trying to download Mojave, that is why they were in the App Store.

The OP must have tried to download Mojave before, because the message above means the

App Store has detected a copy or partial copy of Install macOS Mojave.app in the Applications folder.

On the OP's second attempt the App Store would have said 'Open' and not 'Get', clicking on 'Open'

will instruct the mac to open the earlier download of Mojave, therefore if the download is corrupted

in any way you get the message above.

It does not mean the installer that was found has an expired certificate.

If you try to install an expired version of an OS you get a different error message,

which I cannot remember.