Epic Games can't login offline

Solution 1:

According to the official website found here,

Offline mode is only accessible if you don't have an internet connection using the Epic Games Launcher. To enter offline mode:

1. Open the Epic Games Launcher.
2. On the Sign-In screen, click on Sign In Later at the bottom.

That's all you have to do to use online mode. Here's a word of advice though. Some games, such as Fortnite, may not function, due to being Internet-dependent.

Solution 2:

You would need an internet connection upon the first login. This is because the Launcher can't verify that the account details are correct, as it has no idea what the Epic Games Server has. It would also have no way of knowing what you own without an internet connection.

Basically, sign in with an internet connection, then your credentials will be valid.