x是dict。与Javascript不同（我认为），在dict中访问给定键的值和访问属性的值之间存在语法差异。

x["foo"]  # Value associated with key "foo"
x.foo  # Value associated with the attribute "foo"

您的循环需要前者：

def won(list):
    result = []
    for x in list:
        if x["status"] == "Won":
            result.append[i]
    return result

您可以通过列表理解非常容易地做到这一点：

def won(lst):
    return [comp for comp in lst if comp['status'] == 'Won']

print(won(john_williams_compositions))

或者，您可以使用filter()函数：

def won(lst):
    return list(filter(lambda comp: comp['status'] == 'Won', lst))

print(won(john_williams_compositions))

因为你是一个初学者，列表理解对你来说可能不是很容易，这里有一个简单的实现：

def won(list):
    result = []
    for x in list:
        if x['status'] == 'Won':
            print(x['status'])
            result.append(x)
    return result

print(won(john_williams_compositions))

你只需要遍历字典列表，然后选择那些status键等于Won的字典。
也许最好写一个函数来处理任何状态值。就像这样：

john_williams_compositions = [
    {"year": 1973, "Title": "The Poseidon Adventure", "type": "Best Orignal Score", "status": "nominated"},
    {"year": 1974, "Title": "Cinderella Liberty", "type": "Best Original Score", "status": "nominated"},
    {"year": 1974, "Title": "Tom Sawyer", "type": "Best Original Score", "status": "nominated"},
    {"year": 1975, "Title": "Earthquake", "type": "Best Original Score", "status": "nominated"},
    {"year": 1976, "Title": "Jaws", "type": "Best Original Score", "status": "Won"},
    {"year": 1978, "Title": "Close Encounters of the Third Kind", "type": "Best Original Score", "status": "nominated"},
    {"year": 1978, "Title": "Star Wars: Episode IV - A New Hope", "type": "Best Original Score", "status": "Won"},
    {"year": 1979, "Title": "Superman", "type": "Best Original Score", "status": "nominated"},
    {"year": 1981, "Title": "Star WArs: Episode V - The Empire Strikes Back", "type": "Best Original Score", "status": "nominated"},
    {"year": 1983, "Title": "E.T. the Extra Terrestrial", "type": "Best Original Score", "status": "Won"},
    {"year": 1983, "Title": "If We Were in Love", "type": "Best Original Song", "status": "nominated"},
    {"year": 1985, "Title": "The River", "type": "Best Original Score", "status": "nominated"},
    {"year": 1988, "Title": "Empire of the Sun", "type": "Best Original Score", "status": "nominated"}
    ]

def get_by_status(lst, status):
    result = []

    for d in lst:
        if d.get('status') == status:
            result.append(d)
            
    return result

print(get_by_status(john_williams_compositions, 'Won'))

输出：

{'year': 1976, 'Title': 'Jaws', 'type': 'Best Original Score', 'status': 'Won'}, {'year': 1978, 'Title': 'Star Wars: Episode IV - A New Hope', 'type': 'Best Original Score', 'status': 'Won'}, {'year': 1983, 'Title': 'E.T. the Extra Terrestrial', 'type': 'Best Original Score', 'status': 'Won'}]

注意：get_by_status()可以在一行代码中实现为列表解析。由于OP显然是初学者，因此为了清晰起见，以这种方式编写。

def get_by_status(lst, status):
    return [d for d in lst if d.get('status') == status]