The error message does not say that your use of relative import is wrong:
It says what you try to import is not in your current package. When you execute touch subpackage1/__init__.py you make subpackage1 into a package. You cd into the directory, which means that for import you work from subpackage1, not your original package. So
..subpackage2.moduleZ says go up one directory, which is no longer in the package you are working in, although it would be in package.