Cannot upgrade to OS 10.14.

If you get a corrupted message...

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 2014, 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, restart, then a new copy can be downloaded

Try reinstalling this in Safe Mode, but not using Firefox...

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Safe Boot, (holding Shift key down at startup).

Back up your data, check your applications for compatibility, and download Mojave or Catalina. The information in the article applies to Big Sur, not those.

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Choose Applications from the Finder's Go menu, throw the installer away, and download another copy.

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OK, take care my friend. :)