While the announcement for the Steam Deck may have come as a small surprise, Steam’s user base has ballooned to more than 120 million monthly users in 2020, so it probably shouldn’t have been a big shock to see Valve’s servers falter when presented with such a huge influx of reservations.
For those currently still trying to reserve a Steam Deck, I have some advice: If you’re able to get to the point where you enter your payment information, when you finally confirm the purchase and you see the Working button, just let it go and don’t refresh. In my own case, I was actually able to snag a reservation despite closing my browser tab entirely after the Working button was cycling for about five minutes, so make sure to keep an eye on your inbox for an email confirmation.