In what way did einstein question his own scientific achievements?

Since I don't know the context of this question, I'll answer it the way a scientist would.
We know that Einstein was a scientist. Scientists are always asking questions and creating tests to solve problems. Einstein most likely didn't view his achievements as the answer to the problems of the universe. He probably saw a lot of flaws in his work. Even if he didn't, a true scientist is always questioning things. The definition of a scientific law is a theory that has never been proven false. He probably wanted to know if his achievements were theories or if they were laws.
These are just a few ideas. Take a minute to think about the context of the question and what you think that the answer should be based on that.

1920 census

the census was scheduled for january the 1st, as the government believed that more farmers would be in their homes by that time instead of april. the census did not ask about unemployment, the number of children, and how long was a couple married were not taken into consideration. individuals were considered as residents of the places they normally slept, not the places were they worked at or visited.

2010 census

the census was scheduled for april 1st, 2010. it was the shortest census in history: it only contained 10 questions related to name, age, gender, ethnicity, relationship, and whether the citizen owned a home or rented one. the government launched a website in order to provide the citizens with information regarding the census. there were also "teach census weeks" in schools nationwide.