A XML can have only one top-level ("root") element (<?xml> processing instructions not included). In Hadoop's core-site.xml, this tag should be <configuration>. Thus, try the following:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="configuration.xsl"?>
<configuration>
    <property>
        <name>hadoop.tmp.dir</name>
        <value>/app/hadoop/tmp</value>
        <description>A base for other temporary directories.</description>
    </property>
    <property>
        <name>fs.default.name</name>
        <value>hdfs://localhost:9000</value>
        <description>The name of the default file system. A URI whose scheme and authority determine the FileSystem implementation. The uri's scheme determines the config property (fs.SCHEME.impl) naming the FileSystem implementation class. The uri's authority is used to determine the host, port, etc. for a filesystem.</description>
    </property>
<configuration>

follow the step to install the hadoop: STEPS TO INSTALL HADOOP IN DISTRIBUTED MODE STEP 1:-

#cd /home/masternode
# mkdirhadoopinfra
# chown -R root:roothadoopinfra
# chmod -R 777 hadoopinfra

STEP 1:- Open the /root/.bashrc file and add the following lines in the file

export HADOOP_HOME=/usr/lib/hadoop_package/hadoop-2.5.6
export HADOOP_MAPRED_HOME=$HADOOP_HOME 
export HADOOP_COMMON_HOME=$HADOOP_HOME 
export HADOOP_HDFS_HOME=$HADOOP_HOME 
export YARN_HOME=$HADOOP_HOME
export HADOOP_COMMON_LIB_NATIVE_DIR=$HADOOP_HOME/lib/native 
export PATH=$PATH:$HADOOP_HOME/sbin:$HADOOP_HOME/bin

STEP 2:- Now apply all the changes into the current running system. source ~/.bashrc Now Starting Hadoop Configuration STEP 3:-You can find all the Hadoop configuration files in the location “$HADOOP_HOME/etc/hadoop”. You need to make required changes in those configuration files according to your Hadoop infrastructure. $ cd $HADOOP_HOME/etc/hadoop In order to develop Hadoop programs using java, you have to reset the java environment variables in hadoop-env.sh file by replacing JAVA_HOME value with the location of java in your system. gedit hadoop-env.sh and add java path to this file export JAVA_HOME=/usr/lib/hadoop_package/jdk1.8.0_161

STEP 4:- core-site.xml The core-site.xml file contains information such as the port number used for Hadoop instance, memory allocated for the file system, memory limit for storing the data, and the size of Read/Write buffers. Open the core-site.xml and add the following properties in between the and tags.

<configuration>
<property>
<name>fs.default.name</name>
<value>hdfs://localhost:9000</value>    //  hdfs://192.168.64.131:9000   <master ip>
</property>
</configuration>

STEP 5:- hdfs-site.xml The hdfs-site.xml file contains information such as the value of replication data, the namenode path, and the datanode path of your local file systems. It means the place where you want to store the Hadoop infra.

<configuration>

<property>
<name>dfs.replication</name>
<value>1</value>
</property>
<property>
<name>dfs.name.dir</name>
<value>file:///home/masternode/hadoopinfra/hdfs/namenode </value>
</property>
<property>
<name>dfs.data.dir</name>
<value>file:///home/masternode/hadoopinfra/hdfs/datanode </value >
</property>

</configuration>

STEP 6:- yarn-site.xml This file is used to configure yarn into Hadoop. Open the yarn-site.xml file and add the following properties in between the , tags in this file.

<configuration>
<property>
<name>yarn.nodemanager.aux-services</name>
<value>mapreduce_shuffle</value>
</property>

</configuration>

STEP 7:- mapred-site.xml This file is used to specify which MapReduce framework we are using. By default, Hadoop contains a template of yarn-site.xml. First, you need to copy the file from mapred-site,xml.template to mapred-site.xml file using the following command. $ cpmapred-site.xml.template mapred-site.xml Open mapred-site.xml file and add the following properties in between the , tags in this file.

<configuration>
<property>
<name>mapreduce.framework.name</name>
<value>yarn</value>
</property>

</configuration>

STEP 8:- Verifying Hadoop Installation The following steps are used to verify the Hadoop installation.

start-all.sh

jps

This problem is XML syntax problem as explained by @Amadan. Counter check your core-site.xml keenly. For my case, I got this error because I had declared, <configuration></configuration> twice in the file.