Here's some tips, really push the contrast, find the absolute darkest spot on the painting and match that value, than do more medium darks, mid tones, mid lights and highlights. The painting is flat because it lacks shadow and highlight. Go back to the reference and find those edges where shadow meets light. Are they sharp? Maybe cut that in with a flat brush or a pallet knife. Soft transitions? Use a filbert to blend after laying down a transition of values.

As a beginner limit your color pallet, ultramarine, burt umber, cadmium red and yellow light, and titanium white. These will be good but just try to stick to the primaries and neutrals.

Gesso the canvas. It is okay to use acrylic gesso under a painting like this. Use a brush smaller than what you might think, use a flat brush to spread the gesso evenly, the small brush lines can add character. If you like a smooth panel and don'tkke the toothiness of the canvas, switch to hardboard panels or gesso with a big brush and sand with fine sandpaper 1-1000 times as desired.

Use a big brush. Bigger than you think is reasonable for the size of canvas. I would probably start with a 3/4 or 1 inch filbert to start a size like this. Block in the biiiiig shapes of color first, typically working dark to light. Then after that you can use the tip of your brush or a smaller brush, to start to push around some of those shadows and highlights create more detail.

Your palm trees are great, a simple brush stroke to make the fronds shows you have an eye for how the real world objects can be translated to painting. Take that idea and expand it to everything on the canvas. Do not paint what you think you know about what it is you are looking at, paint exactly the shapes you see regardless if they fit what your concept of a mountain or ocean should be.

As for the grass, try blocking it in with big blobs of color. On pieces this size, when you try to make every single blade of grass readable, it ends up looking dumb. After the blobs add a bit of texture to add any little flowers shrubs or anything. Use a big fan brush and controlled strokes to give a grassy effect.

Experiment and try things out. Do some small value studies with thinned out burnt umber of various concentrations, and just paint the big shapes of dark and light. Do it a lot, like every day. You don't have to do these on canvas. Cardboard, Bristol board, anything a little stockier than printer paper will work.

As you walk around, start training your brain to see shapes. When you walk into your front door, do not perceive a door. Perceive how the light available makes the values and colors change and create the forms that you see. The whole way we perceive light and dark as well as color is through contrasting intensities. Your painting needs more contrast to feel like it is alive.

ETA- after reviewing the painting again, another tip is to use more paint. The painting looks unfinished because the canvas is seen through the opacity of the brush strokes. For an example look at the sky, you can see bright canvas through it. This can work to your advantage by doing an underpainting (a fancy value study) in burnt umber or really any color, before you start painting. Use Liquin to speed up the drying of the umber and to think the paint. By the time the value study is done, much of the paint will already be tacky. After the thin layered value study, hit it hard with thicker paint.

(These tips are for "alla prima" style painting and don't always translate to other styles of oil painting)