How to fix a constraint problem

Hi Tian,

The bone is flipping and Blender is adding an Empty because it 'thinks' it's helping.
This happens when you add a constraint with the hot-key but you don't have the target bone selected.
Blender says "Oh you want a constraint, I'll give you a constraint, and an Empty as a target".
The bones are reorienting to match the rotation of the Empty (that's why they are flipping)

Try to select the Target bone - "Who want's a constraint?"
Then select to owner - "I do!"

You can delete that empty, (and the constraint) and try it again.

I can also see that you have called your Armature "DEF_Armature".
This can happen when you try to rename a bone but you accidental have it set to Object.

Believe it or not, all these "mistakes" are actually normal and expected.  It's part of your learning ;)

Keep going!