I need to download iMovie and it won't let me because it says I need the 12.6 upgrade.

You did not provide the exact model of your Mac which you can get by clicking the Apple menu and selecting "About This Mac". FYI, You should always provide this information when asking for assistance.

Then see whether macOS 12.x Monterey is even compatible with your Mac. If so, then you can download and install Monterey. However, if macOS 12.x or 13.x is not appearing as an option in the Software Update System Preferences, then most likely your Mac is not compatible with macOS 12.x. For laptops you need a 2015+ model (Pro or Air), but need a 2016+ if just a MacBook (no "pro" or "air").

macOS Monterey is compatible with these computers - Apple Support

If your Mac is compatible with macOS 12.x or 13.x, then you can access the macOS installer from links within this Apple article:

How to download and install macOS - Apple Support

Otherwise follow @VikingOSX's advice. If you never previously downloaded iMovie for Big Sur, then you won't be able to do so now. Not sure if it will allow access to older versions from previous versions of macOS or if they will even run on Big Sur.

If the criterion for iMovie is currently macOS 12.6, then you will not get that version in macOS 11.7.8 by any means. However, if you previously installed iMovie via the Mac App Store, you can launch that application, sign in with the identical Apple ID that you used to install iMovie, and press cmd+0 (zero) to open the purchased panel.

You should then see iMovie on that Purchased panel with an update button or a blue outline cloud icon symbol,

and in any case, just click that and do not cancel any dialog. Eventually, it will ask if you want the last compatible version of iMove for Big Sur, and you allow that.