This file would go generally in your documents/sports-interactive/Football-Manager-2024/Editor-Data folder. then you should be able to activate it in-game.
I'm not entirely sure what exactly has happened here. I've put another copy of it here.
Do you have anything that might be affecting the team names or something like that. I'm unsure if the OP changed some of the Serie A-C team names to their IRL ones. If so, then that would cause conflict with any team name change modifications you have active. The only other thing that I can think of is that potentially there's a clash with another competition or league. If you use any other files, let me know, and I can get around to creating a separate file that might be able to be compatable?
Hey, sorry for my lack of response.
Hopefully, this upload works.
Italy (Terza Categoria-Serie A).xml