Here's why:
* Predicates require a verb: Predicates are the part of a sentence that tells us what the subject is doing or being. They always contain a verb. This sentence lacks a verb.
* Incomplete thought: The sentence doesn't convey a complete thought. It doesn't tell us what the boy is doing at the store.
To make this a complete sentence and identify the predicate, we need to add a verb. For example:
* The boy ran to the store. (Predicate: ran to the store)
* The boy walked to the store. (Predicate: walked to the store)
* The boy went to the store. (Predicate: went to the store)