One reason for this issue is that the Object.assign method is not compatible with Internet Explorer. To resolve this, you will need to use a polyfill. You can find a working polyfill on Mozilla's website.
The placement of the polyfill in your code matters. It should be loaded before any other scripts. Consider putting it in a separate JavaScript file and loading it before your other script files as shown below:
<script>
... Polyfill script
</script>
<script src="react-app.js"></script>
If you are building a React application, it is likely that you are using a bundler like webpack. In this case, it is advisable to use babel to transpile your code to ES5 for better browser compatibility. Babel offers additional features and benefits that you may find useful. I recommend exploring more about it.