MacOS High Sierra
Cannot upgrade from High Sierra to Mojave.
Error message: This copy of the Install macOS Mojave.app application is damaged, and can‘t be used to install macOS.
iMac (Retina 5K, 27-inch, 2017)
iMac Line (2012 and Later)
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Silly Apple Error messages...
Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...
Disconnect from the Internet & set the Date back to 2015, then you can run the Install MacsOS Mohave App in Applications, so expired Certificates are not expired.
Or... trash the Install MacsOS Mojave.app in Applications then a new copy can be downloaded.