Hi Aurora,
thank you for your post and kind words.
The problem you are having is that you are using an old version of QtFuzzyLite and then using probably the latest version of the fuzzylite libraries. The property LockValid was renamed to LockPrevious from version 5.0. If you rename that property in your FIS file, you should be able to import your engine.
However, I want to encourage you to use the FuzzyLite Language (FLL) instead of any other format. Not only is the FLL much
nicer and simpler, but also provides features of fuzzylite that are not available in other libraries. For example, LockPrevious is a property that only works in fuzzylite, and matlab will not import the FIS file because it does not know what to do with this property. (Same happens with LockRange).
Cheers.