Checking-in is designed so no data is lost when an item has been edited by multiple back-office admins.
If an admin opens something to edit like an order, fixed fare route, car etc. then other admins will see the message you have posted if they try to edit and save the same item.
Sometimes admins do not Save or Close the item and either close the browser or use browser's back button and this makes the item "locked" until they properly Save/Close it.
Here you can find more information about this: https://booking.drivenot.com/en/knowledge-base/83-how-to-fix-error-check-out-failed-with-the-following-error
You can always unlock an item by selecting it in the list and clicking on the "Check-in" (Unlock) button in the toolbar.
Please try that and if you still have a problem let us know by specifying which section you see the message in - cars, orders, fixed routes, settings etc.
Hope this helps.