Why do I get SQLAlchemy nested rollback error?

The problem was solved.
The point, in this case, is that rollback is not executed until we call rollback explicitly, so when we include commit(), we should write it in a try statement, and write rollback() in the exception statement (in most case) as written in https://docs.sqlalchemy.org/en/13/faq/sessions.html#this-session-s-transaction-has-been-rolled-back-due-to-a-previous-exception-during-flush-or-similar

And, here is the correct code example. I quoted this from the link above.

try:
    <use session>
    session.commit()
except:
    session.rollback()
    raise
finally:
    session.close()  # optional, depends on use case