That means you have created at least one delivery note. Manager will not allow a tab to be disabled if there are existing transactions in it. Does your left navigation pane actually show a zero in the Delivery Notes counter window, as @Brucanna said it must?
If you have existing delivery notes, you can use Batch Delete to get rid of them. But that will have other consequences. So be careful. Read the Guide: https://guides.manager.io/16464.
Before making any decisions, you should also read the three-part Guide on inventory management, beginning here: https://guides.manager.io/11111.
Depending on your usage and update history with Manager, you may be caught in a more complex situation. Originally, delivery notes were effectively just packing slips. They did not affect calculations of quantity on hand. And the concept of delivery not occurring until one was completed had not yet entered the program’s design. When goods receipts were added to the program, the purpose of delivery notes changed and, as you saw, the concepts of quantities to be received, to be delivered, on hand, and owned were either added or revised.
So, if you previously were using delivery notes as packing slips, your software update has now told the program you have not delivered all those old orders. Let us know if that is your situation. The recovery could be more complicated. I won’t fill up this post with instructions unless I know they are necessary.