Can I play Smoke and Mirrors while only having one unit in play?

No, you cannot play Smoke and Mirrors if you only have one unit in play.

According to the card text and RiftJudge FAQ #9739, Smoke and Mirrors requires you to "Choose a unit you control and another unit you control at a different location." Because this is a requirement to play the card, you must have two such units available to target in order to put the spell on the chain. If you do not have at least two units at different locations, you do not have the required targets and cannot play the card.