The negative of 'can' is either 'can't' or 'cannot': we don't write 'can not' in modern English.
In fact, if you try to type in 'can not', most spellchecks will correct it to 'cannot'.
The only time you would see this as two words is if it's imitating emphatic speech. For example, speaker might separate the two words in speech to "You can NOT do that!", putting a lot of stress on the 'not'. However, the standard, unemphatic, form here would be 'can't'.