I got Mac OS X 10.7.5
You might want to refer to the following KB for installation guidance.
As advised by the error, did you try to re-download the installer and check that your internet connection is working while you try to install?
If you still cannot install our product, you might want to open a support ticket so that our agents can help you better.
I tried this and it works. Main question is: how this is even possible with F-secure?
A) To uninstall the F-Secure Antivirus for Mac
Copy your product key from manage subscription
1.Open the folder where you installed the product. By default, the product is in the Applications folder.
2.Open the F-Secure folder.
3.Double-click the Uninstall F-Secure Anti-Virus for Mac icon. The uninstallation program opens.
4.Click Uninstall . You need to enter your administrator password to uninstall the product.
5.Enter your administrator user name and password and click OK .
To install the F-Secure Antivirus for Mac