It is absolutely okay to not love every single detail of your job. When answering this question it is best to be specific. You will want to avoid brushing a broad stroke and saying something like 'I dislike documenting' - since that is a huge part of your role as an OTA.
"There are so many aspects of the job that I love so much, and they outweigh any negative. If I could pinpoint one thing, I would say that I disliked administering electrotherapy in the previous clinic I worked at. The equipment was a bit intimidating to me as it was older but I worked through it with the assistance of my Occupational Therapist."