Hadoop and Python MapReduce - subprocess failed with code 127

I'm running a Hadoop cluster using this repo https://github.com/big-data-europe/docker-hadoop and trying to run the MapReduce job
I'm running the command: hadoop jar /opt/hadoop-3.2.1/share/hadoop/tools/lib/hadoop-streaming-3.2.1.jar -file ./mapper.py -mapper mapper.py -file ./reducer.py -reducer reducer.py -input /purchases_input -output /purchases_output and have Error: java.lang.RuntimeException: PipeMapRed.waitOutputThreads(): subprocess failed with code 127 Why does it happen? How can I find some logs to understand what's going on?
Appreciate any help