Your system needs PC4-21300 DDR4 2666 MHz
[https://support.apple.com/en-us/HT205040|iMac memory specifications]
I generally recommend buying RAM either directly from the supplier or from a major store. Buying from Amazon or eBay can be very iffy!
As an example here’s the Crucial web site for the RAM your system needs [https://www.crucial.com/usa/en/ct16g4sfd824a|Crucial 16GB DDR4-2400 SODIMM]. I was looking just the other week for some SODIMM’s for a customer and Crucial was out of stock then as well. I would make a call to Crucial, don’t forget Crucial is a memory manufacture. The demand for these SODIMM modules is stripping supply.
So is your source real or fake? Do they really have them in stock? So be careful!