An apostille is a certification set up by a treaty, to streamline the recognition of documents that were created in one country but used in another. The text of the treaty is available online. I don't know the specific reason it can't be used within a country, or even if the assertion Hedgehog mentioned is even true. But certainly the treaty establishing apostilles was not signed for the purpose of getting documents recognized within the country where they were created.
If you describe the type of document, the US state where it was created, and the US state where it is to be used, someone may know what requirements exist to make the document acceptable in the state where it is to be used. It may also matter what agency will be receiving the document; an office of vital statistics might have different requirements than a court.