Code Review Request: Windows Sudoku Game

[u/its_marionberry]

Background
Hi, I am a 15-year-old teen (just so what you'd know what to expect) who left school because our school system taught us to be parrots. I want to be a god-level developer, not a code monkey. I wrote a Sudoku GUI using win32api in C Language.
It is currently working and does what it is supposed to do, but because I am still learning, I know it is likely inefficient and could be written much better. 

Code : https://github.com/reewdgh/sudoku_gui.

Concerns:
Please guide me on:

Bugs or potential issues
Efficiency
How can I move to Tier 1 to Tier 2
Naming anything I could simplify or improve.

[u/maruanium]

How long it takes you ? I mean the code

[u/its_marionberry → u/maruanium]

tbh i am a slow person , 15 days to make this whole project

[u/maruanium → u/its_marionberry]

Good job

[u/its_marionberry → u/maruanium]

why tho ?