Are you sure you are not talking about the [m]flushleft[/m] environment? Which package is the [m]\leftflush[/m] command from?
This site uses MathJax, which, as I understand it, implements primarily the part of LaTeX used for typesetting formulas and not text. Do you want to align text?
Does the \leftflush command work on this site? Yea or Nea?
There is no command known as [m]\leftfllush[/m]. [m]\flushleft[/m] command is the correct one. Even if the command is accepted by MATHJAX, it might not give pleasing results. I suggest you to use the left align button available while posting.