Sharks are saltwater fish and require a large body of saltwater to survive. Lakes are freshwater bodies of water, and sharks cannot survive in freshwater environments.
It's possible you've heard about Lake Nicaragua in Central America, which is sometimes referred to as the "shark lake". However, the sharks found there are actually bull sharks which are a type of euryhaline shark, meaning they can tolerate both saltwater and freshwater. They are able to survive in the lake because it connects to the ocean via the San Juan River.
So, while there may be some sharks in Lake Nicaragua, it is not a lake that has sharks in the true sense of the word, as there are no lakes in the world that exclusively have sharks.