"Installation cannot proceed because the installer is damaged"

It's got to be frustrating. I am guessing that the reason you are unable to successfully install your operating system is because the installer became corrupted during the download. Put the installer into the trash and do another download. If you were using Wi-Fi to do the download, don't. It's too much data for Wi-Fi. Instead, you need to use an Ethernet cable directly connected to your modem/router. If necessary, temporarily move your computer to do the download. And during the process, stay off the internet. If you are unable to directly connect to the internet, then be prepared to do multiple Wi-Fi downloads until you get a good one that loads. And allow each download to fully complete, which can take up to an hour, more or less, plus another 45 minutes or so for the installation. Last, if you are connecting to a download link, use Safari, not Firefox. Good luck. I hope it goes well.

Start up in Recovery Mode (Restart then hold Command - R keys). First open Disk Utility and run First Aid then quit that and run install MacOS.