If you are using Sandbox and getting this message "No command 'hdfs' found" then it means you might not have done the SSH to Sandbox on port 2222 (which is mandatory for these commands to work). Similar issue reference
# ssh firstname.lastname@example.org -p 2222
# hdfs dfs -ls /tmp
If you are getting this message in a Non-Sandbox environment then you must make sure that the "HDFS_CLIENT" package or component is installed on that host. You can install the clients on the host from Ambari UI
Ambari UI --> Hosts (Tab) --> Click on Desired Hostname --> Click on "Install Clients"