It took a long time for me to get through this game but I managed.

i playd through the entire game. so what do i think.
well at the start of the game you should put some sort of instructions. eg what to do and how you do every thing. also tone down some of the upgrade caps (EG) the jump upgrade. change that from 20 to 10 to make it a more easy quick game. you should also probably t least make the player sligtly moveable at the start.
so yeah i had fun through the entire game so i rate this a 4.

how can i get more coin?

Why is that ground monster killing you every time you start?

Two big problems: time spend on collecting money and not being able to progress:

There is too much time you cannot even try to progress because you don't even have the chance to. Of course the concept of the game is needing to buy the upgrades to be able to progress but if you have to die 20 times or so without even having the chance to explore a new area it gets annoying and boring. Also the bossfigt depended too much on the bullet upgrade, i started trying it with 10 bullets and I think you need 25 or so to beat it. At least give it a health bar so you know whether you should even try it.
Some upgrades can even have a disadvantage, for example the jump upgrade because you get used to your previous jumping hight. I also waited a lot before uprgrading the time upgrade because you are really trapped at the beginning so you just wish for time to run out after you have colleced all the acessible coins.
Make the player earn money quicker the further they progress and make it hard but almost always possible to progress further.
The game has a lot of potential but those issues kill the fun!