its because it renders the page before the request and the items is an empty array you can set the result inside your useEffect like this:

  useEffect(() => {
    fetch("https://test-front.framework.team/paintings")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setItems(result);
          console.log("paintings");
        }
      );
    fetch("https://test-front.framework.team/authors")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setAuthors(result);
          console.log("authors");
        }
      );
    fetch("https://test-front.framework.team/locations")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setLocations(result);
          console.log("locations");
        }
      );

  const result = items.map((elem) => {
    return {
      author: authors.find((author) => author.id === elem.authorId).name,
      created: elem.created,
      id: elem.id,
      imgUrl: elem.imgUrl,
      location: locations.find((location) => location.id === elem.locationId)
        .location,
      name: elem.name
    };
  });
  }, []);

or you can set a boolean state when your request is done like this and then using jsx map your items:

import "./styles.css";
    import { useEffect, useState } from "react";

const App = () => {
  const baseURL = "https://test-front.framework.team/";
  const [items, setItems] = useState([]);
  const [authors, setAuthors] = useState([]);
  const [locations, setLocations] = useState([]);
  const [hasItemsArrived , SetHasItemsArrived] = usestate(false)

  useEffect(() => {
    fetch("https://test-front.framework.team/paintings")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setItems(result);
          setHasItemsArrived(true);
          console.log("paintings");
        }
      );
    fetch("https://test-front.framework.team/authors")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setAuthors(result);
          console.log("authors");
        }
      );
    fetch("https://test-front.framework.team/locations")
      .then((res) => res.json())
      .then(
        (result) => {
          setIsLoaded(true);
          setLocations(result);
          console.log("locations");
        }
      );
  }, []);

  
 

  return (
    <div className="App">
      {hasItemsArrived && items.map((elem) => (
        <div key={elem.id} className={"item"}>
          <img src={`${baseURL}${elem.imageUrl}`} />
          <li className={"cardInfo"}>
            <ul>{elem.name}</ul>
            <ul> {elem.author}</ul>
            <ul>{elem.created}</ul>
            <ul>{elem.location}</ul>
          </li>
        </div>
      ))}
    </div>
  );
};

export default App;
 

There is an issue with getting the value for image element from items array to result. I have updated your code sandbox and all the required attributes are displaying correctly. Please verify.