You have the following and the last three lines have the wrong indentation:
last_calculation = "{0} {1} {2} = {3}".format(num1, choice, num2, result)
print(last_calculation )
past_calc.append(last_calculation)
else:
print("Unrecognized operation")
They need to be changed to:
....last_calculation = "{0} {1} {2} = {3}".format(num1, choice, num2, result)
....print(last_calculation )
....past_calc.append(last_calculation)
..else:
....print("Unrecognized operation")
NOTE: I have used dots to represent spaces, as the formatter is destroying my indentations.