The hype around the latest Fortnite update was huge enough to get the entire community excited to play the game the moment the new update dropped for Chapter 3, Season 4. However, since a large number of players have been trying to log into the game since the time the update went live, a bottleneck was only natural.
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The current "waiting in queue" message that you see right now can be attributed to the sheer number of players trying to log into Fortnite currently. There is only so much load that a server can handle, and this "waiting in queue" message can be termed as a failsafe that's in place to prevent the servers from crashing.
Moreover, the official Fortnite Status Twitter mentioned that they were working on solving an issue that was preventing players from logging into the game. So this "waiting in queue" error could be attributed to server-side issues as well. For now, all you can do is wait for the developers to fix any ongoing problems. Also, keep an eye on the Epic Games status page for updated information about Fortnite server status.
For more Fortnite related content, check out Where to find the Cobra DMR in Fortnite and more, on Pro Game Guides.