I agree with @MeHeartCruising, your English is very good but the NCL wording is not so clearly written.
The FCC will definitely be put in your account at the end of your sailing, August 15, 2023. However, as I understand it, it has to be used on a sailing that ends before August 15, 2024. As your 2024 sailing does not end until after this date, I believe you would not be able to use it. I would change your cruise to an earlier one.