Did you try right clicking on .exe and run as administrator?
See, this is one of the real-world problems of a piece of software wanting to install itself into the Program Files(x86) folder. FSInn wants to write a file and Windows says "no you are not going to do that."
This is exactly why I push for nothing FS related to be installed into that folder.
See if FSInn can be installed into a custom path like C:\FSInn
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