Cypress doesn't find Environment variable from cypress.env.json

I have the following folder structure:

tests
  cypress.env.json
  cypress
    fixtures
    integration
      test
        test.spec.ts
    plugins
    support

I have missed out the for the question unnecessary files.
So my cypress.env.json looks like this:

{
  "loginTestStage": {
    "admin": {
      "username": "value1",
      "password": "value2"
    },
    "user": {
      "username": "value3",
      "password": "value4"
    }
  },
  "vatid" : {
    "x1": "value5",
    "x2": "value6",
    "x3": "value7",
    "x4": "value8"
  }
}

Now I try to access the env variable in the test.spec.ts file:

describe('Login', () => {
    const usernameInput = '[formcontrolname="username"]';
    const passwordInput = '[formcontrolname="password"]';
    beforeEach(() => {
        cy.visit(url)
    })

    it('Allow users with valid credentials to login', () => {
        const { username, password } = Cypress.env('loginTestStage').admin;
        cy.get(usernameInput).type(username);
        cy.get(passwordInput).type(password);
        ...

    })
})

When I try to run the test I always get this error here:

Cannot read properties of undefined (reading 'admin')

So the value from the Env variable can't be found. How can this be solved?
Thanks in advance