问题是在第一次循环迭代时返回一个值，这将退出整个函数，而您希望累加result中的值的字符串，然后在函数结束时（循环外）调用return result。
你也可以简化你的代码，就像这样：

def countries(countries_dict):
    result = ""
    for lst in countries_dict.values():
        result += str(lst) + '\n'
    return result[:-1]

print(countries({"Africa": ["Kenya", "Egypt", "Nigeria"],
                 "Asia":["China", "India", "Thailand"],
                 "South America": ["Ecuador", "Bolivia", "Brazil"]}))

最简单的迭代代码基本上只有两行，并且不进行函数调用：

for x in {"Africa": ["Kenya", "Egypt", "Nigeria"], 
             "Asia":["China", "India", "Thailand"], 
             "South America": ["Ecuador", "Bolivia", "Brazil"]}.values():
    print(x)

注意“values（）”方法调用。
如果您更喜欢使用函数调用，则以下代码将实现相同的结果：

# define the function (printing happens in here)
def countries(countries_view):
        for x in countries_view:
            print(x)

# call the function, no printing at this level, printing is done in the function
countries({"Africa": ["Kenya", "Egypt", "Nigeria"], 
             "Asia":["China", "India", "Thailand"], 
             "South America": ["Ecuador", "Bolivia", "Brazil"]}.values())

请注意，它不是“print（countries...）”，而只是“countries（）”。