This is because the bone is connected to its parent, which is normal.

1. Change to Edit mode
2. Make sure the same bone is selected
3. Under Relations as in your screenshot, uncheck Connected
4. Go back to Pose mode

Now you can give the bone an absolute location.

EDIT: If you expect to move a bone and have the connected parent bones follow, you could use IK (Inverse Kinematics). You can easily add this as a modifier on the bone.

If you want a bone to have a parent bone you have to do the parenting while in 'Edit Mode'. At the minute you are in 'Object Mode'. From the header of the 3D view, with the armature selected, choose 'Edit Mode':

(The bone you want to be the parent will also have to be in the same armature for it to be available as the parent).

You need to be in Pose mode to have access to the constraints.

I guess you mean the way to change parent of a bone: