This copy of the Install macOS Mojave application is damaged and can‘t be used to install macOS.

If you get a corrupted message...

It may not be damaged, it's might be that you have an old copy already in Applications, there are 2 cures...

If you’re short of bandwidth or data… Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS ******* App in Applications, so expired Certificates are not expired.

Or...

Trash the Install MacsOS *******.app in Applications then restart, a new copy more recent version can be downloaded.

apparently the downloaded file was damaged during the download and it cannot be used. I suspect you used Wi-Fi to do the download. Instead, use a wired Ethernet connection and it should go well. Wi-Fi is too slow for these very large files. If you cannot use a wired connection you may have to do multiple downloads to get a file that can install.

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

(253705)

Ronasara wrote:

apparently the downloaded file was damaged during the download and it cannot be used

Certificates used to sign installers, Web sites, etc. often have expiration dates. The idea is that even if someone cracks the security, the expiration date will limit the damage (by causing the attack to expire with the certificate). With luck, the expiration date will occur long before there is any successful attack.

Unfortunately, this sometimes leads to Web sites that can't be verified (because their maintainers neglected to get updated certificates) – or old copies of Mac OS installers becoming "damaged". Not because they are "damaged" but because they are no longer considered secure.

+1 on trying Niels suggestion first. That specific wording almost always means the current installer found an older, outdated version of itself already on the computer.