Saving Issue for Forza Horizon

Whenever i try to Save rehash and resign my Forza Horizon car setup file, i get the error " An error has occurred while writing back to the loaded package" & then it says “The process cannot access the file (my file) because it is being used by another process”. I just bought diamond two days ago because i wanted to be able to mod Forza Horizon and not being able to is getting me a little irritated. Thank you if someone can help.

(And this is only happening with this file because with my profile & other items it saves rehashes and resigns fine. I do not know what’s wrong.

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