In some cases, you can, and it usually does give you a boost of energy or motivation when you need it most. However, only tapping on the positive doesn't address the root causes of the distress you may be feeling.
Think of it like moving new furniture into a dirty house. Sure it may look good, but the dirt is still there under everything you just put in. So it's usually best to clean everything up first (tapping on the negative - or the "truth" about how you feel) and then tap in the positive affirmations afterward.