"Cannot" is nearly always written as one word. I think brmadeira is right about it being a grammar checker: it probably assumed that since the sentence began with "Following", you intended it to be in the past tense.
Spellcheckers can be useful, but they don't know everything and they often mark words correct that are in reality wrong; grammar checkers are, in my experience, completely useless.
"Can't", like all contractions, is generally used in informal situations, although the use of contractions is becoming more widespread. These days, the long forms are reserved for the most formal of situations: newspapers, technical documentation, official records, formal business letters, that kind of thing.