When I am trying to run " echo" command from Execute External Command Step,I am getting "java.io.IOException: Cannot run program "echo" " error.
There is no program called "echo". Output into a shell is provided by a shell function called "echo", so you would always get a "can't run program" error if you try to invoke it.
What are you trying to output, where are you trying to output it, and why? When I need to output anything, I tend to use the testExec class in a JSR-223 step, as it can output to log files and to the test execution results. Various examples of the use of this class can be found in DevTest 8.0 - Scripting Guide - V1.1.pdf
Rick is correct. "echo" is a shell command and not a program. If you want this to work, you need to select the "Exec Shell" checkbox on the step: