Check the folder
Kits live in your Documents folder, not in the game's install folder. Football Manager 27 reads them from:
A common mistake is to drop packs into steamapps\common\Football Manager 27. That folder is for the game itself (and for BepInEx plugins); graphics there are ignored.
Check the file names
Football Manager finds a kit by the club's ID and the kit type in the file name. The ID is the one the game uses for that club, for example 322 for Flamengo.
- OK
322_home_3d.png,322_away_3d.png,322_third_3d.png - OK
322_gk_home_3d.pngfor the goalkeeper - NO
Flamengo home.png,322-home.png,322_home_3d (1).png
Copies with "(1)" at the end are easy to miss: they come from downloading the same pack twice.
Check the config.xml
The game only shows a graphic when the config.xml in its folder points to it. There is one record per file: from is the file name without the extension, to is where Football Manager uses it.
<!-- one line like this for every kit in the folder --> <record from="322_home_3d" to="…"/>
If a file is in the folder but not in the config.xml, or the config.xml points to a name that no longer exists, the kit won't show. Writing these lines by hand is where most errors happen.

Look for duplicates
Two files for the same club and kit type means only one of them wins, and it may not be the one you want. Packs from different creators often include the same kits, so after a few years most graphics folders have the same kit two or three times.

Restart the game
In Football Manager 27 you no longer need to reload the skin. Close the game and open it again after adding kits, and they are there.
Still nothing?
Post a screenshot of the folder (with the file names visible) and of the config.xml on Discord. Or skip all of this: the Kits Hub app installs kits with the right names, folders and config.xml for you.
Next guideHow to organise your FM27 graphics folder