Alright, it's possible that one of the sticks is defective, even if it's brand new straight from the package (I've had this before). Depending on the specific issue, it might prevent you from posting or running memtest, among other things (had this as well). Jumping forward, however, in my opinion, with Ryzen, there's no real need to overclock them - simply setting a profile should suffice.
You might consider running memtest with two sticks at 2666mhz to see their performance and eliminate any issues. After that, try setting them above 2666 but below 3200 and test again <-- It's hard to give a definitive answer here. I've had sticks that wouldn't allow me to post, and I've also had a set similar to yours that eventually began working. If I recall correctly, I had to update the BIOS or something along those lines, which is also a weird solution IMHO, as your sticks are from G skill-b-dies, which are well-regarded and of a good standard <-- have been this way for long enough time. It's difficult to provide specific advice, but I'd recommend to run the test above. It's already somewhat a good thing that they function at 2666mhz, though there might be an issue with the generic profile, even if there ideally shouldn't be.
You might also consider lowering the voltage to see how they would behave, as this could likely be the primary concern (maybe...) However, it's still hard to definitively rule out other potential issues at this point.
Also try sticking them in different sockets, and also consider there could be problem with mobo etc
As mentioned, I once had a brand new RAM set, and after spending a lot of time troubleshooting, I ended up returning them because one of the sticks was defective right out of the brand new box <-- It was hard for me to come to terms with such scenario, though.
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