What Was Einstein’s Biggest Blunder?

Humans had always been asking themselves how the universe began, but for the great majority of history, we didn’t have the right mathematical and physical instruments to consistently describe it.
Things changed only at the beginning of the 20th century when Einstein formulated his theory of general relativity.

According to this theory, the fabric of space-time was curved by gravity; time was not universal, but relative – as well as space. In addition, the speed of light in a vacuum was the same in all reference frames.
General relativity was also later shown to predict the bending of light due to a massive object and the existence of black holes. Over the years, this became the best theory we have to describe the world we live in.

As soon as he introduced his theory, Einstein began testing the universe but, in doing so, he made what he called his “biggest blunder” ever. But what was this blunder? And was it an actual mistake?