It's best to transport the refrigerator in the upright position to keep the oil in the compressor where it should be.
When you transport the refrigerator on it's back or side then the oil drains out of the compressor and into the refrigeration lines.
But it is okay to transport the refrigerator on it's side or back but after transporting the refrigerator on it's side or back you need to stand it upright and leave it upright for around 8 hours before plugging it in to allow all the oil to flow back into the refrigerator compressor because if the oil is not in the compressor you'll burn out the compressor on the refrigerator.