The left shift key on my new K2 squeaks in a pretty annoying way. Do you have any recommendations?

Please try the steps below to fix this:

1) Something with the little black stabilizer seat is touching the aluminum plate of the keyboard. You can adjust the placement of switches, stabilizers, and the keyboard plate to avoid this issue.

2)Lube usually helps. Please refer to the video: https://youtu.be/Qot8W0ZNwyM

3)Sometimes it can fix itself after some usage as usage can make it more smooth.

4) Remove the keycap and reinstalling it might help as well because something might not have been installed properly.

Hope it helps.