The path to artifact should be a full relative path from the build folder root, not just a file name. When you put it like path: PrisonRanks.jar AppVeyor is looking for artifact in the root of build folder - I guess you should have something like path: your\output\bin\folder\PrisonRanks.jar.
The root folder can be replaced by a default placeholder like $root?
Right now i don't know where the artifact is going, but from my build.gradle should be in the root folder where it do the script.
So when it clone the repository, it says this:
git clone -q --branch=master https://github.com/GABRYCA/Prison_Ranks.git C:\projects\prison-ranks
the root folder it's C:\projects\prison-ranks ? Or C:\projects\prison-ranks\Prison_Ranks