There seems to be a misunderstanding in your question. Albert Einstein did not believe that the Earth was flat. In fact, he made significant contributions to our understanding of the curvature of space and time through his theory of general relativity.
Einstein's theory of general relativity proposed that the force of gravity is the result of the curvature of spacetime caused by massive objects. This theory provided a new understanding of the nature of space, time, and gravity.
The idea that the Earth is a sphere has been well-established for centuries, dating back to ancient Greek philosophers and further supported by scientific evidence such as satellite imagery, photographs of the Earth from space, and the observations of Earth's shape during circumnavigations and space missions.
Einstein's work focused on explaining the fundamental principles of the universe, and his theories have been extensively tested and confirmed through experiments and observations. It's important to note that his research and contributions were not related to asserting or challenging the shape of the Earth.