GUI
increase play window
remove settings word, the gear icon is enough
fix the colored font, it's unreadable. A small 1-2px black outline might work, with slight increasing the font
GUX
unable to buy button, should have some sound failure horn.
WASD+Arrow keys+Mobile, to work simuntaneously
Game pause via key, is always helpful
your plane should have some sound, or at least smoke effects line as he flies
timer should be top, not distractive as is now
There are lots of other things to comment, or improve. Destroying planes with rockets, might be fun for boosting the score, while speakign of it, what is the purpose of your game? Is it plane challenges? If so, the name and style in playing, should be towards hinting that. Remember, challenges don't like forever, so you might want to tweak your logic. Otherwise it's a simple evader with minimal effects.
How long you made this game, and what engine? It looks the top pause bar recognizable to me..
Good luck!