If you are on a Team plan but you are not the Team plan owner, you can't publish new prototypes for your team from the Craft Sync plugin. First, you need to create a new team prototype in the InVision web app. You can then return to the Craft Sync plugin and sync your Sketch or Photoshop file to that prototype.
If you are the Team plan owner, new prototypes will always be published to the Team plan. This cannot be changed.