cannot load F Secure total because running old Mac with High Sierra

Hello,

Sorry for my reply. I am only an F-Secure user.

I think it is not officially possible. And, also, design of installer is with autoupdating functionality (so, it should try to install the latest available build of F-Secure TOTAL app, anyway).

Perhaps, their official view is that they cannot guarantee any proper work on unsupported platform (version of system there) or it is technically impossible. And they cannot offer old version of F-Secure app - because it may contain critical bugs, which fixed later and so on. Thus, completely limited way to provide a security for unsupported platform (especially, if support is dropped by OS-developer).

Discussed, for example, there: Old version for OSX 10.11? — F-Secure Community (and some other topics are available).

And KB-article: I cannot install F-Secure Internet Security (old SAFE) on macOS version 10.14 (Mojave) or older - F-Secure Community

But you can try to reach their official Support (for example, via web-chat): Contact support | F-Secure

However, I think there is no offline installer available; and online installer should upgrade itself to recent one and to install current app build (is not possible; because will fail on system compatibility check).

Maybe you can try to ask for refund or partial refund (based on your circumstances);

Or to use licenses on your other devices. If F-Secure app is installed on any other 'already' not supported platforms - try to not reinstall or uninstall it. So, at least, it will be stuck with its version til possible to receive AV database updates (for example).

Thanks!