Your Mac is compatible with macOS Big Sur, Catalina, and Mojave.
For Mojave and Catalina upgrades, click on the following blue text and read: How to get old versions of macOS - Apple Support
For Big Sur, click on the following blue text: macOS - How to Upgrade - Apple Support
You will need to have all your apps 64-bit—use this Go64 tool—for upgrades to Catalina and Big Sur.
No 32-bit apps—Microsoft Office 2011 and earlier, for instance— will work on Catalina and later. Find each 32-bit app that you’re currently using, and decide what to do; upgrade, retire, or migrate.
If you are using Office 2011 or earlier for instance, you’ll need an upgrade to Office 2016, Office 2019, the upcoming Office 2021, an Office 365 subscription, or a migration to an alternative such as LibreOffice or Apple iWork.
Mojave can run older 32-bit and newer 64-bit apps. But it’s the last macOS release that can do that, as the old interfaces have been retired.
I’m running mostly Catalina clients, and various of the latest Apple software such as iWork supports Catalina at the oldest, as you’ve found here with iMovie.
In addition to updating or finding alternatives to your 32-bit apps, have a backup or two, prior to performing any upgrades. If you have no current backups, you’re one hardware failure or one software failure or one command error away from catastrophic data loss.
As for the iMovie app download on your current High Sierra, if you clicked the download button as would be expected from that message and the older-version download presumably then somehow failed, use the Get Support link in the upper right and discuss this with Apple. If you didn’t click the download button, click that. You’re in an older version and Catalina is required for the latest version of iMovie, as happens with some other Apple apps.
