There were at least two major contradictions in the reign of Catherine the Great.
First, there was a contradiction in her religious policies. On the one hand, she issued an edict of toleration that said that people of all faiths had to be tolerated in her realm. On the other hand she infringed on the rights of Jews in Russia in very important ways.
The idea of tolerance was very much of an Enlightenment idea, which brings us to the second major contradiction. Catherine was very interested in Enlightenment ideas. She corresponded with some of the most famous Enlightenment thinkers such as Voltaire. At the same time, though, she went completely against Enlightenment ideas by trying to create an absolutist monarchy. This was the central contradiction of her reign.