Hi Jeewon,
It looks like you are using Rigify, so there are 2 ways to control the IK pole vector.
If you are NOT using a pole vector, you need to animate to the rotation on the first segment of the IK arm. Maybe you hide the control in your rig but it’s the stupidly massive double arrow shape. Animate the Y rot on that control and it will control the direction of the elbow.
If you ARE using the pole vector, you need to animate the pole vector control after clicking the button in the UI to display and enable it.
If it is a problem with the actual generated rig, go to your metarig and change the limb from “Automatic Rotation” to either X or Z (depending on which way you need it to bend.) Then regenerate.
This should force it to bend as intended, although you need to make sure you actually have a bend in the elbow in edit mode.
Hope this helps.