Does it give different nuance if I say contraction “didn’t” instead of “did not”?
There isn't a huge difference between using "didn't" or "did not." However, because the speaker is refuting a statement, he has likely chosen to not use any contractions. This helps to add emphasis to the fact that he was not able to do something (successfully write a good thesis paper). Ex: A: Did you lie about taking the book? B: I did not. Ex: A: Do you have any toilet paper left in stock? B: We do not.