Apple may have sent a warning yesterday to its Retail Store managers and other resellers stating that it could not fulfill new orders for its desktop Macs, possibly due to low iMacs and mini Macs stocks, with updates approaching. According to AppleInsider, all iMacs are affected, as well as the most basic mini Macs.

Citing the Amazon portal as an example, such products would all be unavailable at the moment, and can only be purchased through third-party resellers affiliated with the company.
In the past few days, we've seen some rumors pop up about new iMacs (and even about MacBooks), but nothing has been said, for now, about a new generation of Macs mini. According to the THERE, the SKU (Inventory Maintenance Unit) for your model entry-level it would have even been discontinued by Apple.
Information from the “Buyer’s Guide” of MacRumors point to imminent updates on both desktop lines: the iMac was renewed 210 days ago, and has an average cycle of 220. The Mac mini, in turn, had its last update along with the iMacs, but has an average cycle slightly longer, 252 days.