What is the result of using a non-double-faced card in an attempt to transform it?

When a player attempts to transform a card that is not a double-faced card, the effect simply fails, and nothing happens. The rules governing card transformation are specifically designed to apply only to double-faced cards, which are the only types of cards that can transform under the right conditions. Non-double-faced cards do not have a transformation state defined within their mechanics, so when an effect is triggered that specifies transforming a card, and that card is not capable of doing so, the intended transformation cannot occur.

As a result, there is no alternative action, such as sending the card back to the deck or discarding it, and there is certainly no instant transformation into another state like a creature. Since the effect stipulates that only a double-faced card can transform, the outcome is that the attempt is unsuccessful, indicating that nothing happens, aligning with the choice you identified.