X399 taichi recognizes only 24G out of 32G memory

Just chiming in to say I had the same problem, and the same solution.  I have an x399 Professional Gaming motherboard, Threadripper 2990wx, and 4 x 8GB Kingston HyperX 2400 DDR4 (HX424C15FW2K4/32).  Switching out RAM sticks didn't work.  Unplugging the PSU and mashing the Clear CMOS button for ten seconds, then upgrading to BIOS 2.2, then to 3.3, didn't solve the problem.  (My computer would just power cycle continually with 2.2 as it couldn't recognize the Threadripper 2--I had to use the Flashback button and a USB stick without powering on [cool feature!].)  Then I took off the cooler and TIM, raised and lowered the TR4 bracket down as straight as I could, reapplied TIM and cooler, and ? moë! Problem solved!  Thanks!

You cant find it because he forgot the frist 2 letters

Try running it at higher 3000 or lower 2400 MHZ

ASRock memory support does not state any 2666 RAM Support from GSkill

If nothing works send it back and order a new one from the Supportet list

Very odd.

1. Even Google hasn't heard of this F2666 model of yours. Now, I'm not a googling guru but still, 0 hits on that model? Are you sure you haven't been scammed?

2. Check the manual for populating the proper RAM slots. http://asrock.pc.cdn.bitgravity.com/Manual/X399%20Taichi.pdf

Check and even better, re-seat the RAM modules in their respective places. DDR4 slots are known for misleading novice users. RAM must be seated 100% correctly in the proper slots.

3. Check if CPU is seated properly and it doesn't have bent pins. Some Foxconn TR4 socket mechanisms are known to be very difficult to install properly. It's also another common mistake, even for advanced builders. Even better - re-seat the CPU once again. I've seen guys having similar issues to yours, where bent pins or CPU misalignment was the culprit.

3. Check for cabling issues - bad plugs, forgotten secondary PSU cables, etc.

4. Do a proper CMOS reset. I know 'proper' sounds stupid but many times it's not just as simple as a push of a button. Dig around here. There are good guides. Or just ask, I'll write as soon as I can.

5. Try running each RAM module solo. It should be in slot A2 but try others if it won't work there. Clear CMOS every time! From BIOS select 'Load XMP Setting' and choose 'XMP 2.0 Profile 1'. Save and exit. Boot Win, Lin or memtest and test extensively. Rinse and repeat for each RAM module.

6. Although at this point it should be clear (at least to you) if your memory or something else is wrong, it's worth checking https://www.jzelectronic.de/jz2/index.php and flash the latest beta. Always reboot once, then power off and clear CMOS, before and after flashing new BIOS.

7. Last but not least, check the QVL https://www.asrock.com/mb/AMD/X399%20Taichi/index.asp#Memory

It states that many G.Skill modules can run @ 3200 (and above) in Quad Channel. Maybe it's time for a swap?

Keep us posted! Fingers crossed!