Error message while downloading Mojave

Try this. Check the Applications Folder and see if the Mojave installer app is there. If it is drag it to the Trash. Enter your administrative password if needed to trash it. Then download a fresh copy of the Mojave install app.

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

If you get a corrupted message...

Naw, it's not damaged, it's just 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 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.

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