Unfortunately, for the jsonix schema compiler, Java 8 is required. It will not run on more modern versions of Java.
Check to make sure you're running Java 8: java -version "java version "1.8.0_201"
Then, install Node dependencies: yarn install
java -jar node_modules/jsonix-schema-compiler/lib/jsonix-schema-compiler-full.jar -d mappings -p PO $YOUR_XSD_SCHEMA_FILE_HERE