If you're trying to start an entry but can't see any categories in the category drop-down menu then it's likely to be a browser-specific issue.
Please try the following troubleshooting steps:
Try forcing a hard refresh of your browser window
For Windows, hold down the Control button on your keyboard and click the browser refresh button
On Mac hold down the Shift button on your keyboard and click the browser refresh button
If refreshing your browser doesn't fix the issue, disable any browser extensions you may be using. Some extensions can interfere with elements on a web page.