Yes, you can turn it on. If you have a solar-only install and are referring to the small black box that is the solar gateway, you probably won't see any system monitoring data in your app until Tesla activates. However, it should still work (and, if it is sunny, you should be able to tell it is working by looking at your utility meter.)
If you have PWs and are talking about the energy gateway, then everything should have been working when they completed the install and you should see monitoring as soon as you turn everything on.