When new content launches in Destiny 2, it is common to experience server outages or login issues that prevent players from entering the game. Online errors are apt of many live-service online games, and Destiny 2 is no different, which is exactly why the player receive a message that Destiny 2 is at capacity.
When Destiny 2 is at capacity, this means that Bungie's servers cannot support the amount of players that are attempting to play the game. New expansions, seasons, and events like Solstice of Heroes often usher in tons of new players and returning players, which can overload Bungie's servers when such a large amount wants to play at the same time.
There is no way to fix this error from the user's end. Since the capacity issues occur on Bungie's end, it is up the development team to solve the error. During this maintenance period, players must wait for Bungie to fix the issue before logging on again. This usually takes a handful of hours or, at most, a day.
Reading the "at capacity" message upon logging in to Destiny 2 might be surprising for frequent players since this error is less likely to occur mid-season. As seasons progress, many players drop out of online games like Destiny 2 due to multiple factors: boredom, or a separate, new game releases that draws attention away. So when an influx of players return for a new content drop the servers, understandably, exceed capacity.
Bungie is often reliable about reporting any server issues or scheduled maintenance time, especially upon a new content drop. If you are experiencing issues, check out the official Bungie Help account over on Twitter for real-time updates. Or, check out the Destiny Server and Update Status page on Bungie's website.
For more Destiny 2 guides, check out Destiny 2 Chicken error code explained and solutions and What is the Weasel Destiny 2 Error Code? on Pro Game Guides.