Delete button disappears when I select an item and click on it

You don’t see the delete icon anymore because nothing is selected in the canvas (you have deleted the item you selected)

When you select an element in the Smartscene editor the system displays features that apply to the kind of element you selected. This is why you see the edit features appear by the right and re-shuffle by the top menu. And this also explains why the edit options by the top keeps changing as you select objects in the editor.

If nothing is selected in the editor, you won’t see the delete icon and some other edit options.

If you select an object, the edit options by the top will reshuffle (based on the kind of object you selected)  and reveal. The edit menu by the right will also reveal.