Here's why:
* Prepositional phrases consist of a preposition followed by its object. The object can be a noun, pronoun, or another phrase.
* In "on a piece," "on" is the preposition, but "a piece" is a noun phrase, not the object of the preposition.
To make a prepositional phrase using "on," you need an object that is being related to the location "on." For example:
* on the table
* on the wall
* on the phone
In these examples, "table," "wall," and "phone" are the objects of the preposition "on," and the entire phrase describes a location.