You should probably clean up the previous build attempt but I don’t think it would cause problems.
The CMake build is designed to be run out-of-tree (i.e. from a build directory). Creating build/release and/or build/debug is optional, but it’s not a good idea to just run cmake in the root of the source tree as it will make it very hard to work with Git.
The script does not create any build directory for you, which is why the build instructions include a step for you to run mkdir build/release and change to that directory before running cmake