Hey, I've tried to build your project myself, because I wanted to add the ores for lead, silver and tin, but I have this issue whenever I tried to build the project:

Any idea how to fix it? (I am kinda new to Java/Fabric modding as a whole)

Try to run the "build" or "jar" task of the gradle project

If you mean to run the project in a fabric client environment, that is working, I am trying to "export"/build the mod so I can't use it my server, etc.

for some reason, there isnt a /jar folder into the fabric/build directory

The compiled mod should appear in fabric/build/jar directory

Okay, I manage to find the jar file, it is not where it is supposed to be, but its there and working.
I have another question tho, how can I change the fabric API version without breaking the whole mod? Because when I do change the fabric api version, this is what I get.