Complex sentence: The reason was that Cabot who was assigned to go to the Spice Islands never went there but explored the Sierra de la Plata which was a violation and as a punishment he was exiled for two years to Oran.
Split into simpler sentences: The reason was that Cabot who was assigned to go to the Spice Islands never went there but explored the Sierra de la Plata which was a violation. As a punishment he was exiled for two years to Oran.