Did you place it in the correct spot? Make sure it is .pak (not .zip) so you might have to extract it from the download if you use WinZip, 7zip, or other programs that change a download into a zip file. (You will need to create a ~mods folder in the Paks folder, where you will extract the .paks file to.)
My ~mods folder address is D:\SteamLibrary\steamapps\common\ACE COMBAT 7\Game\Content\Paks\~mods
tho yours might be C:\SteamLibrary\steamapps\common\ACE COMBAT 7\Game\Content\Paks\~mods
If you got the game from a different program/site/DVD then you will have to find it in your C drive and it might be under a folder labeled games
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My ~mods folder address is D:\SteamLibrary\steamapps\common\ACE COMBAT 7\Game\Content\Paks\~mods
tho yours might be C:\SteamLibrary\steamapps\common\ACE COMBAT 7\Game\Content\Paks\~mods
If you got the game from a different program/site/DVD then you will have to find it in your C drive and it might be under a folder labeled games
Link: https://www.moddb.com/games/ace-combat-7/addons/girly-air-force-j39e-f15c-f4e