If you are getting the error "java_home is not set and could not be found" when trying to start the Hadoop master node, it means that the system cannot find the Java executable required to run Hadoop.
There are several possible causes for this error:
Java is not installed on the system: In this case, you will need to install Java and set the JAVA_HOME environment variable to the location of the Java installation.
JAVA_HOME is not set correctly: Make sure that the JAVA_HOME environment variable is set to the correct location of the Java installation.
The java executable is not in the system PATH: Make sure that the java executable is in the PATH environment variable so that the system can find it.
To resolve this error, you will need to troubleshoot the cause and make sure that Java is properly installed and configured on the system. You may also need to update the Hadoop configuration files to point to the correct location of the Java executable.
For more information about installing and configuring Java for Hadoop, you can refer to the Hadoop documentation (https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/SingleCluster.html#Installing_Java).