Fix Cart Functionality in Calculator

I'm experiencing an issue with my calculator's cart functionality where items are not adding to the cart. Whenever I try, an error message pops up saying "Error."

Your task includes:
- Diagnosing the issue causing the error message
- Fixing the cart functionality so items can be added successfully

Ideal candidates for this project should have:
- Strong experience with troubleshooting cart functionality
- Proficiency in debugging error messages
- Familiarity with calculator coding would be a plus

Please let me know if you can assist with this. Thank you!