Q:

What suitable sentence to explain the item cant be deleted? 

I want to tell the user that this item cannot be deleted because this is the only item left in the list. 

A:

Error message: "You need to have at least one item in this list." 
Additionally, you can communicate this (im)possibility leveraging on the the Delete button. 

Grey out the Delete button (that automatically sends the signal)
If they still attempt to click on it, you can then show the message.