Yeah more or less the comments are correct, but let me clarify! FILE0 is looking for an environment variable, that contains your API key. This is how you authenticate yourself from your backend. When you want to use client uploads you can use f0.createToken('myfile.png') and send that token to the frontend, where you can also import f0 and use it like this: f0.useToken(myTokenFromBackend).set(blob) In the dashboard you will find a setup guide and code snippets with all this info.