A ideia é não permitir likes negativos, já que dislikes estão aí pra isso. Se for o caso isso pode ser alterado depois. Também alterei a função de add_like() para não haver hardcoding de ocorrências. Para esse add_likes() trabalhar junto ao @likes.setter, ele precisa de um novo self.likes ao final dele. Também usei um método estático para formatar números abaixo de dez para temporadas de séries; caso o número seja maior que dez, tal número será retornado.

Por fim, recomendo usar o VS Code com extensões como AREPL for Python, que facilitam muito a vida.

class Movie:

    def __init__(self, name, year, length):
        self.__name = name.title()
        self.year = year
        self.length = length
        self.__likes = 0

    #getter properties
    @property
    def name(self):
        return self.__name

    @property
    def likes(self):
        return self.__likes

    #setter properties
    @name.setter
    def name(self, new_name):
        if new_name is None or new_name == "":
            raise ValueError("Gotta have a name!")
        else:
            self.__name = new_name.title()

    @likes.setter
    def likes(self, new_amount):
        if not isinstance(new_amount, int):
            raise TypeError("Amount of likes needs to be an integer.")
        elif new_amount < 0:
            raise ValueError("There are no negative likes.")
        else:
            self.__likes = new_amount

    #instance methods
    def add_like(self, occurrences):
        if occurrences < 0:
            raise ValueError("Can't decrement likes.")
        new_likes = self.__likes + occurrences
        self.likes = new_likes


class Series:

    def __init__(self, name, year, seasons):
        self.__name = name.title()
        self.year = year
        self.seasons = seasons
        self.__likes = 0

    #getter properties
    @property
    def name(self):
        return self.__name

    @property
    def likes(self):
        return self.__likes

    #setter properties
    @name.setter
    def name(self, new_name):
        if new_name is None or new_name == "":
            raise ValueError("Gotta have a name!")
        else:
            self.__name = new_name.title()

    @likes.setter
    def likes(self, new_amount):
        if not isinstance(new_amount, int):
            raise TypeError("Amount of likes needs to be an integer.")
        elif new_amount < 0:
            raise ValueError("There are no negative likes.")
        else:
            self.__likes = new_amount

    #instance methods
    def add_like(self, occurrences):
        if occurrences < 0:
            raise ValueError("Can't decrement likes.")
        new_likes = self.__likes + occurrences
        self.likes = new_likes

    #static methods
    @staticmethod
    def format_small_numbers(number):
        numbers_to_words = {
            1: 'one',
            2: 'two',
            3: 'three',
            4: 'four',
            5: 'five',
            6: 'six',
            7: 'seven',
            8: 'eight',
            9: 'nine',
            10: 'ten'
        }
        return numbers_to_words.get(number, number)

avengers = Movie("Avengers", 2009, 160)
avengers.add_like(3)
avengers.name = "the AveNgers"
print(
    f"{avengers.name} is a movie from {avengers.year} with a {avengers.length}-minute runtime. Likes: {avengers.likes}",
    "\n")

sopranos = Series("Sopranos", 1999, 6)
sopranos.add_like(2)
print(
    f"{sopranos.name} started in {sopranos.year} and ran for {Series.format_small_numbers(sopranos.seasons)} seasons. Likes: {sopranos.likes}",
    "\n")