Einstein never said this, it was falsely attributed to him in a self help book “The Rhythm of Life: Living Every Day with Passion and Purpose” by Matthew Kelly 2004, this is the first time the quote is mentioned in relation to him.
I appears in no collections of Einstein's works or quotations, but something similar does appear in an early treaties on higher education by an anonimous author as quoted in 1940 by a Prof. Dolbear of Tufts college.

I hope beyond hope that Albert was not so insensitive as to make such an antipiscatorial comment. As a matter of fact there are fish that do climb trees, and very well, and fish are great flyers as well.

There are lots of flying fish, while many birds fly little or none. Hence, we can make a general statement:

FISH ARE BETTER FLYERS THAN BIRDS.

There is a popular myth that due to genetic differences birds fly better than fish, but that myth needs to be dispelled. It is true that some studies have shown fish as a group to be deficient in flying, but that is clearly due to early childhood conditioning, peer pressure, and social expectations. To help fish achieve their due status as flyers, new laws must be enacted to protect them from the bigotry of piscaphobic individuals.

Because of negative associations, henceforth that awful F word (f__h) is to be avoided. The new word will be "Aeroquat". In future, anyone who uses the F word will find himself demonized and banished from polite company.

Textbooks must be rewritten to reflect the new meme. University courses in Aeroquatic awareness must be required.

News outlets, such as this one, must stand constant watch so as to enforce proper thought and speech patterns on the uneducated working under-classes in regard to our Aeroquatic citizens.

"Here Are 6 Things Albert Einstein Never Said"
[history.com]

As if the fish cared what anyone thinks! ???