How can I delete the item?

When I delete an item in Sales Invoices, I get an undeletable the item as shown in the figure below.

How can I delete the item ?

What item are you trying to delete?

The page contains an overdue sales invoice, which may contain an item. And you cannot delete a line from a posted invoice.
If you need to do that, then you must create a credit memo and a new invoice, which you can do from the posted invoice page.

PS: I have moved this topic to the users forum, which is where we have questions regarding the application.