Finally, I figured it out. The root cause is that I was maintaining two versions of Java on my machine i.e. 1.7 and 1.8 although PATH and JAVA_HOME variables were pointing to 1.8. By trying different options with PATH and JAVA_HOME variables, I ran into further problems - Now, I was no more able to even run JMeter UI by double clicking jmeter.bat file as I started facing same error (Initially, I was able to run .bat file by double-click). I uninstalled and re-installed Java 1.8 but issue stood still.
On further investigation, I realized that maintaining two versions could be the root cause and on doing further research I came to know that I have to remove all java.exe, javaw.exe, and javaws.exe from Windows\System32 and Windows\SysWOW64 folders. By following this solution, my issue is resolved completely and testplan is being executed as scheduled task.
Ref: Registry key Error: Java version has value '1.8', but '1.7' is required