How can I download Mojave OS? It is not showing in the app store anymore and my computer is too old to upgrade to Big Sur.

https://support.apple.com/en-us/HT211683 Open Only In Safari and Safari will redirect to Open the Apple Apps Store and download Mojave.

Good Luck and Best in the New Year

Click on the text "macOS Mojave 10.14" in the Related Article above, using a web browser other than Firefox.

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This copy of Install MacOS Mojave application is damaged, and can’t be used to install macOS.

That means there is already an older and outdated copy of the installer in your Applications folder. That file is what you need to delete before proceeding.

delete it and use Ethernet instead