title: Add-in Error: Something went wrong and we couldn't start this add-in.
body: <p>I am using Create React App to build an add-in for Excel. The add-in works well in the Excel for macOS.</p>
  
  <p><kbd><a href="https://github.com/Hongbo-Miao/office-addin-react" rel="noreferrer">The Repo</a></kbd></p>
  
  <p>However, when I try to load it in Excel Online. After uploading the manifest file and opening the add-in, it shows</p>
  
  <blockquote>
    <p>Add-in Error: Something went wrong and we couldn't start this add-in. Please try again later or contact your system administrator.</p>
  </blockquote>
  
  <p><a href="https://i.stack.imgur.com/fkBcG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/fkBcG.png" alt="enter image description here"></a></p>
  
  <p>The console shows </p>
  
  <blockquote>
    <p>Error while parsing the 'sandbox' attribute: 'ms-allow-popups' is an invalid sandbox flag.</p>
  </blockquote>
  
  <p>I am already using HTTPS in development by <code>HTTPS=true npm start</code>.</p>
  
  <p>What else may cause this? Thanks</p>