I like Bob Ross quite a bit myself, he inspired me to get into painting. I see you said you are adapting his style/using as a launch point - as you branch-out into your own. That's actually much like how I spring-boarded into my own style; he has tons of good advice of great use when it comes to the mechanics of the tools and mediums. so with that foreword here is my best critique of what you did well and what you could take a second look at: [Disclaimer: I am only a hobbyist so take this all with a grain-of-salt, it is merely my attempt to help you develop your art]

As was mentioned already Gesso, Sky, and color depth (a bit of lighter or even some sap green highlights on the forward trees could 'pop' the depth well - even in a cool-toned painting like this). Cutting in a far-shore (maybe with some Umber) will give a better transition of the water into the land. The reflection on the water has some dissonance, you pulled the trees down pretty well for a calmer-water look that is carried by the surface of the lake; but the lack of reflection of the mountains disrupts that sense of clam (trying to add the mountain reflection without a sky would present its own set of issues though). The palette knife breaking-technique is done successfully in several parts, I advise to be very patient and attentive to this movement as you learn. The tallest peak in particular shows signs that you shifted method; and it leaves you with edges that looked worked toward a vanishing point. Not bad in itself, and there are places that method could be of use, but the eye sees a bit too much order; and mountains tend to have more of an organized chaos way about them (still it is not impossible for a mountain to look like that either){also I can imagine the walls of the box may have presented the need for a variety of tricks with the palette knife}. So that's pretty much it; all in all it is a good early attempt and on a rather a-typical surface. I hope some of this helps and is not terribly off base lol. Keep creating, and always keep enjoying the process.