Are there any actual keyframes and if so, on what properties? What version of AE? What system? You need to provide more info.
That is Source Text keyframe and it can only be a hold keyframe. There is nothing to interpret with a hold keyframe other than hold so it makes sense that the Keyframe Assistant would not function. There is nothing it can do. The same applies to any keyframe that you cannot do anything to but change the value like a checkbox control.
What were you planning to do? if you want to transition from one line of text to another you either have to use two text layers or apply an animator to your text. The only character animators available are Character Offset and Character Value, neither of which would be very useful in creating a word.