There is no direct train from Aksu to Xining Station, why is there no transfer plan displayed?

Yes, it is available on Ctrip, there are transfers in Hami, and transfers in Lanzhou are available in several cities!

You can go to the Baidu subway map to see that there should be transfers. Ctrip online has.

From Aksu to Xining, you can take a direct train to Shanghai, or take a train to Urumqi and then to Xining