I think that your issue is how you've used the lookup function. Look in "helpers.py" to see how the lookup function is defined. The first row of code includes a 'startswith'. Your error message occurs at the first point when you try and use the lookup function.
Consider how your code is trying to access the 'dict' via the lookup function. It is not picking up the data because you are using lookup(symbol) and it is not finding anything at '' because the symbol is referenced in the dict as a 'str' (string).
This link provides a good overview of how a 'dict' works: https://learnpythonthehardway.org/book/ex39.html