Port Issue Connecting React frontend to Flask/Python Backend

We are trying to connect our frontend and backend (made with the help of the articles related to this guide) but can't get the last step to work. The React page on localhost:3000 displays a button that, when pressed, should fetch some info from /profiles in the backend (ran on localhost:5001). It's when this button is pressed that the error message shows up in the terminal (nothing happens in the browser).

The current version of our code is in the features branch in our repo. We have tried adding "proxy": "http://localhost:5001"to our package.json file as well as "proxy": "127.0.0.1:5001". We have also switched between 5000/5001 and edited names of paths, but to no avail!