Go to Configuration > Web and app > Apps
Make sure the web category is added to the app:
If this doesn't resolve the issue, check the "display period" on the web category under Web and app > Web categories. The display period only needs to be filled out if you want the web category to be shown for a limited amount of time, otherwise - leave it blank.
Make sure the "Show", "Show on customer web" and "Show in app" box is ticked.
Note! Because of cookies it can take some time before you see the changes on your computer but the customer will see it immediately. Open your website in an incognito window to see the changes immediately.