The fifa19.key file may not be in the right place. Double-check the steps above. If the issue persists, try using a different mod manager version (1.0.5.9 is widely considered the most stable for FIFA 19).
When you download Frosty Mod Manager (usually version 1.0.5.9 or higher for FIFA 19) and try to apply mods, you might see a dialogue box labeled This is not a CD key for the game itself. It is an internal security and compatibility feature. Frosty Mod Manager Key Fifa 19
Because FIFA 19 runs on EA's encrypted Frostbite Engine, the Frosty Mod Manager needs a special decryption key file to read and modify the game's data. The fifa19.key file acts as a digital skeleton key, unlocking the game's files so the manager can apply mods properly. This is also true for other Frostbite games like FIFA 18 and FIFA 20. The fifa19