Answer 1

The code is written well. However, when I copy and paste your code to the console it says "IndentationError: unindent does not match any outer indentation level".

Check the indentation in every clause; don't use the space bar... Use the Tab key!

Answer 2

Console in playground separate input by space. so if you want to add 7 by 8 you should type 'add 7 8 quit' in input prompt.
dont forget to put quit in the end of input
also try this input prompt
'add 7 8 multiply 7 8 substract 7 8 quit'

Answer 3

The code playground doesn't like to receive input more than once from my experience
Use a fully functional interpreter like Python 3 on your PC or QPython3 for Android.