My answer to this question:
First science fiction story to refer to a real and living (at the time) scientist?
Says that Albert Einstein was mentioned at least twice in the Skylark of Space (1928).
My answer to this question:
First story to mention the speed of light?
Has only one quotation that mentions Einstein by name, in chapter VIII "indirect Action".
Regaining his self-possession as the wisdom of his friend's advice came home to him, Seaton sat down and pulled out his pipe. There was a tense silence for an instant. Then he leaped to his feet and darted into his room, returning with an object-compass whose needle pointed upward.
"DuQuesne did it," he cried exultantly. "This baby is still looking right at him. Now let's go—make it snappy!"[531]
"Not yet. We should find out how far away they are; that may give us an idea."
Suiting action to word, he took up his stopwatch and set the needle swinging. They watched it with strained faces as second after second went by and it still continued to swing. When it had come to rest Crane read his watch and made a rapid calculation.
"About three hundred and fifty million miles," he stated. "Clear out of our solar system already, and from the distance covered he must have had a constant acceleration so as to approximate the velocity of light, and he is still going with full...."
"But nothing can possibly go that fast, Mart, it's impossible. How about Einstein's theory?"
"That is a theory, this measurement of distance is a fact, as you know from our tests."
"That's right. Another good theory gone to pot. But how do you account for his distance? D'you suppose he's lost control?"
https://www.gutenberg.org/cache/epub/20869/pg20869-images.html
Smith wrote The Skylark of Space between 1915 and 1921.
https://en.wikipedia.org/wiki/The_Skylark_of_Space
So 1915 is the earliest year Einstein could have been mentioned while writing, 1921 is the latest year that the mention of Einstein could have been added in a revision, and 1928 is the earliest that magazine readers could have read that mention of Einstein.
So any mention of Einstein in science fiction after 1928 would be later than in The Skylark of Space, any mention of Einstein in science fiction between 1905 and 1915 would be before The Skylark of Space, and any mention of Einstein in science fiction between 1915 and 1928 would be a borderline case.