Error Messages:

Could not execute ruok towards ZK host xxx:2181. Add this line to the 'zoo.cfg' configuration file on each zookeeper node: '4lw.commands.whitelist=mntr,conf,ruok'. See also chapter 'Setting Up an External ZooKeeper Ensemble' in the Solr Reference Guide.

where xxx is the host name or IP address

Solution:
is to add a line 4lw.commands.whitelist=mntr,conf,ruok inside zoo.cfg as stated in the tip

vim zookeeper/conf/zoo.cfg

Add a line at the end:

4lw.commands.whitelist=mntr,conf,ruok

Exit after saving and restart zookeeper:

zkServer.sh restart