When users encountered an unexpected behavior in QuickBooks Online, it's generally recommended to try general browser troubleshooting.
To get started, use a private browser. This is to verify if this is caused by the system or your browser. To do so, you can use the keyboard shortcuts below:
Internet Explorer: Ctrl+Shift+P
Once done, try to create an expense transaction again. Check to see if it works. However, if you're still unable to save, you can clear the cache of the browser that you're using. Cache is stored in order to speed up browser loading times, however, too much can accumulate and impact processes and caused unexpected behavior. For more information about this process, here's an article that you can check: Delete or Disable Cache and Temporary Internet Files in Your Web Browser.
Additionally, to learn more about creating an expense transaction, see below resources: