https://www.gamespot.com/articles/snes-classic-edition-pre-order-guide-tracking-walm/1100-6451207/

August 31: Walmart has begun contacting certain customers to notify them of canceled SNES Classic pre-orders. Based on reports from NeoGAF and Reddit, this only appears to affect those who ordered more than one unit, and it seems only the additional orders have been canceled. (We at GameSpot have also received such a cancelation notice, reducing an order for two units down to one.) There's no official word on this yet, but it's possible this move could free up additional units for pre-order.

August 25 update 2: The SNES Classic Edition is back in stock at Walmart right now. It likely won't last long, so be sure to jump on it quickly if you're interested. This may be one of the last opportunities to pre-order the system prior to its launch in late September.