That statement is often quoted and usually attributed to Einstein. He never said it, though. I also think it's wrong. Feynman said something similar, yet different: "I couldn't break it down to freshmen level, which means we haven't really understood it yet."
I have never heard this quote being attributed to Einstein. DuckDuckGo can't find the quote in the first place; Google does yield a few results and yes, one or two websites indeed attribute the quote to Einstein, but others attribute it to other people.
In any case, are you sure you're not thinking of the much more famous quote
> Everything should be made as simple as possible, but not simpler
which is also wrongly(?, [0]) attributed to Einstein?
