You can easily install this package from npmjs.com where you can find necessary documentations and examples to get started. Installation:

npm i @mollie/api-client

the point here is that the effect is being invoked after the react component mounted and rendered to the user. The next line where you are trying to call Mollie in fact running earlier when component is being constructed but not rendered yet.

There are multiple options what you can do about it:

1. Import script in the index.html file as you do for standard non-React solution. There should be a "public" folder containing this file in case of create-react-app usage or other place in case of custom project setup. The HTML should exist in any form even in case it's being generated on the server side dynamically. A mollie instance can later be created in the component or globally.
2. Use multiple effects in the React component: one to load Mollie and another one to use it when loaded:

// MollieExample.jsx
const MollieExample = () => {
    const [mollie, setMollie] = useState();
    useEffect(() => {
        const script = document.createElement('script');
        script.src = 'https://js.mollie.com/v1/mollie.js';
        script.addEventListener("load", () => {
            setMollie(window.Mollie(/* Mollie arguments */);
        });
        document.body.appendChild(script);
    }, []);
    
    useEffect(() => {
        if (mollie) {
            console.log('Mollie exists here');
            console.log(typeof mollie);
        }
    } , [mollie]);

    return <p>typeof mollie: {typeof mollie}</p>;
};

3. Use dynamic script loading in case it is required with globally shared Mollie instance via custom hook:

// useMollie.js
let molliePromise;
const useMollie = (effect, deps) => {
    const [mollie, setMollie] = useState();
    const mollieCb = useCallback((mollie) => effect(mollie), deps);

    useEffect(() => {
        if (!molliePromise) {
            molliePromise = new Promise((resolve) => {
                const script = document.createElement("script");
                script.src = "https://js.mollie.com/v1/mollie.js";
                script.addEventListener("load", () => {
                    resolve(window.Mollie(/* Mollie arguments */);
                });
                document.body.appendChild(script);
            });
        }

        molliePromise.then((mollie) => setMollie(mollie));
    }, []);

    useEffect(() => {
        if (mollie) {
            mollieCb(mollie);
        }
    }, [mollie, mollieCb]);
};

// MollieConsumer.jsx
const MollieConsumer = () => {
    useMollie((mollie) => {
        console.log('Mollie exists here');
        console.log(typeof mollie);
    }, [/* useMollie effect callback dependencies array */]);

    return (
        <p>Mollie consumer</p>
    );
};

// App.jsx
function App() {
    /* Both consumers use the same instance of Mollie */
    
    return (
        <div>
            <MollieConsumer/>
            <MollieConsumer/>
        </div>
    );
}

I assume you will end up with using some middle option. For instance, with importing script in the index.html (or any other sort of the HTML page you have containing the React application host element) and global hook.