Got the latest version of blender (2.90.1) As you can see, i didn't subdivided any faces more than another, however some faces would correctly merge while some others would stop merging in the middle (picture 1). Even when creating an edge between the two points, the edge would just be raised but not the face, it wouldn't fill the gap in between (picture 2). I also tried pressing F to fix it but it gives me the "canno't merge face" error (please forgive my english). Could someone help me please ? Maybe do i need to use an older version of blender ?
You can't do what you're trying to do with this kind of topology. Actually, in 3D, faces are always made of triangles, for example a quad is made of 2 triangles:
If I move up the top right vertex the line between the triangles will rise up, like you would like in your example:
But if I move the top left vertex it will stay flat:
So in your case, you need to add topology, like that:
To join 2 vertices through a face you need to press J rather than F, F will simply create an edge between 2 vertices over the existing face, it won't cut the face.