I read it in the text below:
Cannot is the correct form in almost all cases. The only (very rare) exception is in sentences like "You can do it, or you can not do it"—in other words, where the two words have their separate meanings ('are able not to'). In the normal sense ('not able to'), it is always one word. Anyone who tells you different is trying to get you red-penciled.
What does it mean? Is it a phrase or expression?
In the US, at least, teachers traditionally corrected papers with a red pencil.* (Today it's a red ballpoint, or a Comment in MS Word .)
(Newspaper copy-editors, however, traditionally corrected with a blue pencil, which would not show up in a photo-lithograph. Later, when copiers came into use, they shifted to red pencils, until computer typesetting enabled them to ruin correct your copy without markup. Today of course, newspapers cannot afford copy-editors.)
So your source is saying that to spell it as two words invites bad marks, and urging you to ignore advice to the contrary.
* My father corrected papers with a blue fountain pen, but he taught at a university. And since no one could read his handwriting anyway it didn't matter.
