To log a message,we can type anything in first part and instead of ERR we can give Warning etc.

${__log(Failure occured due to reasons i dont know,ERR,,)}

We can also call BeanShell fnc within log fnc

${__log(${__BeanShell(str ="Failed";str.concat(" this test");)})}

Then in jmeter we can go to options and click log viewer and in that we can view whether the above data we we typed in logged or not.