The "Encryption Key" error usually crops up in older versions of the Mod Manager or when the software encounters a specific patch of the game it doesn't recognize. Essentially, the Mod Manager is saying:
Frosty Mod Manager (versions 1.0.5.9 and above) is a tool designed to inject custom assets (textures, databases, locale files) into the Frostbite Engine—the engine powering FIFA 18. However, EA Sports routinely encrypts its game executables (the FIFA18.exe or FIFA18_trial.exe file) to prevent tampering. fifa 18 frosty mod manager encryption key
