I am new to blender and started creating a character of an online template some time ago. I am currently on the animating phase and I am making a walk action. I was following an online tutorial [Video]. I was going to paste the mirrored copied key frames but I could only paste it mirrored on the x-axis, and not on the y-axis which is what I need.
To use the Mirror Copy, your model needs to be symetric along the X-axis. To achieve that you have to rotate your model and your armature in Edit Mode and possibly change your Mirror Modifier on your model to mirror along the X axis.