attribute error instead of correct output

[Larz60+]

In your code, def SayHello is not indented (and must be)
this is a difficult code to write interactively, a better example (using a script file) below (includes a default greeting)

MyClass.py:

class MyClass:
    def __init__(self, greeting='there'):
        self.greeting = greeting

    def say_hello(self):
        print(f"Hello {self.greeting}")

def main():
    myinstance = MyClass()
    myinstance.say_hello()
    myinstance.greeting = 'Zelda'
    myinstance.say_hello()

if __name__ == '__main__':
    main()

Output:
Hello there

Hello Zelda