Hello, if you chose "No" when asking for Easy Anticheat to be installed after the Fortnite update, you will need to install it. You will be unable to start Fortnite without this program. The easiest way to install it is to just exit the EAC launcher and run the game again from the Epic Games Launcher and click yes when the EAC install prompt comes up from Windows.
If the prompt doesn't come up for some reason you will have to install it manually following these steps:
Navigate to the Easy Anti Cheat folder in your Fortnite install directory [Default is C:\Program Files\Epic Games\Fortnite\FortniteGame\Binaries\Win64\EasyAnt iCheat Folder]
Locate EasyAntiCheat_Setup.exe
Double click the file
Click the install Easy Anti Cheat button
Start the launcher and Fortnite