Ensure your bike is properly attached to the Snap Trainer.
Some tires are a better choice than others, so if unable to resolve, you may want to change to a different tire.
There are tires manufactured for indoor use, yet they may not grip the roller as well. To reduce slip with any tire, clean the tire and roller with rubbing alcohol.
Also, make sure your tire is not slipping on the roller by adjusting the tension of the roller and inflate to proper pressure.