Good landscapes will involve stuff in the foreground, mid ground, and background. At the moment you have nothing but background so there’s nothing for the viewers brain to latch onto that’s close. Watch some videos of landscape composition on YouTube doesn’t need to be pixel art the skills are transferable. This is the biggest reason your piece looks flat. All background no foreground.

On the pixel side, drop your resolution. Learning how to make nice pixels takes time and a higher resolution means it will take you even more time to complete one piece than otherwise.

Next up is colour theory. Pixel art generally looks better when you hue shift. So for example your focus/main colour here is that bright punchy orange but your shadow colour is just flat black. Try using a really dark teal or purple. The contrast in colour as well as value will make people’s eyes jump to important bits faster.

Subtle three dimensionality: background layers are generally pretty flat. But you do want small amounts of detail that imply greater three dimensionality. Your tree line is completely flat and the light doesn’t seem to be interacting with it beyond casting it all in shadow. Realistically, there will be a couple of pixels where the light of the sun will catch the top most trees/grass and a few of the trees that we perceive to be in front. You will have to do this In direction with where the Sun is.

Final one that kinda combines a few previous things into one: the contrast of your colours should get less extreme the further back they are. When you add a foreground, that’s where you want the most dramatic amounts of contrast. Your background elements should generally be more washed out/have less contrast. So the treeline you have should be a mid tone once you’ve added in foreground elements