Error code 0 is Fortnite's generic failure code, shown when the game cannot launch or cannot sign in without a more specific reason. Because it is generic, the fix is a process of elimination, starting with the launcher and the game files, then the account and the service status.
If error 0 appears only when signing in, try signing in on the Epic Games website in a browser, which shows whether the problem is the account or the launcher. If the website signs in fine but the game still fails, clear the launcher's web cache and try again. If nothing works, check that the account is not locked or awaiting a password reset.
What does Fortnite error code 0 mean? It is a generic code that means the launch or sign-in failed without a specific cause. It covers anything from damaged files to a service outage, which is why the fix works through the common causes in order.
Is error code 0 a ban? No. A ban shows a clear account message, while error 0 is a technical failure. If the account were banned, the Epic Games website would say so when signing in.
Why does error 0 keep coming back after verifying files? Verification only fixes the game files. If the cause is the launcher cache, the account, or a service outage, the error returns until that cause is fixed, so work through the steps in order rather than repeating verification.