经过更多的研究，我发现答案似乎是No React不能读取外部的.env文件。当你构建你的应用程序时，它是“锁定”的（静态的）。阅读这篇文章：create react app cannot read environment variable after build
所以我的解决方案是检查当前的url和动态创建链接到API。类似这样：

export const APIURL = () => {
  //first check if we are in developement mode like localhost:3000
  if (import.meta.env.DEV) {
    return 'http://localhost:8888/mylocation/myapi/';
  }

  //do we have a build?
  if (import.meta.env.PROD) {
    //build the api url dynamic to current location
    if (window.location.hostname === 'localhost') {
      const urlParts = window.location.pathname.split('/');
      return window.location.origin + '/' + urlParts[1] + '/myapi/';
    } 
else return window.location.origin + '/myapi/';
  }

  return 'API-URL-IS-MISSING!';