The problem is that you already have something driving the translateX attribute. In your case, it's being driven by the motion path. When you try to set a keyframe on it, it doesn't let you, as it would destroy the original connection.
Now, you can get around this easily by grouping the object that's connected to the motion path, then keyframing that group. (You'll need to adjust your pivot-point first however) Essentially, you never want any entries in your channel box to be green, which signifies multiple inputs. Sometimes it allows it, sometimes it doesn't. I'm not too sure what the rule is, as I avoid it if I can.
If you have any more questions, please don't hesitate to ask.
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