chapter I differentiated between a text box itself being selected and the insertion point being in the box? Here's one of those cases where it matters. Make sure that the text box itself is selected (it should have a dotted border) before pressing Delete. Pressing Delete with the insertion point inside just deletes whatever character is to the left of the insertion point.
There's a difference between deleting a manual text box and a text placeholder box, as follows:
When you select a text placeholder box that contains text and press Delete, the box will be replaced by a default-formatted empty placeholder text box.
When you select a text placeholder that is empty and press Delete, the placeholder box is deleted.
When you select any manual text box (containing text or not) and press Delete, the text box is immediately deleted.
To get a placeholder text box back that you've deleted, reapply the slide layout (or apply a different slide layout that contains the type of text box you want).