What i try to do

I have a small ReactJS application. In this application, i try to do an REST request to the API of SpreadShirt via the JS Fetch API.

What fails

Basically i get no data. The console of the browser shows the following output:

Access to fetch at 'https://api.spreadshirt.net/api/v1/shops/100749149/sellables?mediaType=json' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status. 

My Code

getData() {
    const header = new Headers({
        "Authorization": "SprdAuth apiKey=\"XXXXXXXXX"",
        "User-Agent": "MukoSoft/1.0 (https://mydomain.de; my@mail.de)"
    });

    const options = {
        method: 'GET',
        headers: header

    };

    fetch('https://api.spreadshirt.net/api/v1/shops/100749149/sellables?mediaType=json', options)
        .then(response => response.json())
        .then(data => this.setState({ data: data }))
        .then(() => this.setState({ loading: false }))
}

Does anyone has a hint for me, while i can't fetch via JavaScript? And can somebody explain, why the request via Postman works, but not via fetch API?

Since this is my first question on stack overflow, you can give me advices on how to formulate questions better