If I try to use /ma I get 'cannot mark, no selection in progress' how do I use /ma to do things like /fixgrass?

 

"/ma" is a shortcut for "/mark all"
All "mark all" does is click at the coords 0 0 0  in the map then click at the coords on the other side of the map (e.g. 127, 127, 127), effectively marking the entire map.

If you're playing in Not Awesome 2, you do not need to use /ma after /fixgrass because it automatically fixes the entire map. If you're playing anywhere else, you should use /ma after you use /fixgrass to mark the entire map.
If you're playing on Not Awesome 2 and you want to only fix a specific area, use /fixgrassarea

Thanks That got it. I was doing /ma before /fixgrass.

Thanks for the prompt reply.