Very good game!
The UI and the gameplay are perfect, however, i think that the Visual aspect can be improved, it would be interesting if you added some type of light system, used the draw_sprite_ext() function in the draw event of the player to add a squash and stretch effect (this type of thing makes the movement feel a lot more smooth and lively), also would be cool to see some particles when you jump and climb.
Besides all that, you could also improve the pixel art and some animations generally, i'd recommend referencing the appearence of other popular games with the same sprite size or aesthetic, and also watching some tutorials from mortmort (i really like his color tutorial) and pixel overload on youtube.
Also if you want to get really good with pixel art i highly recommend checking out some of the art and animation fundamentals; Form, perspective, light, color (these are art), and for animation there is a good youtube video called the 12 principles of animation.
I know this is a bit too much but since you said you have no idea how to improve i decided to write a lot, hope it helps you!
Also sorry if i wrote something wrong, english isn't my native language.
The engrish language option was pretty funny ngl.