_cannot_ is usually preferred over _can not_.

> Both the one-word form **cannot** and the two-word form **can not** are acceptable, but **cannot** is more common (in the Oxford English Corpus, three times as common). The two-word form is better only in a construction in which **not** is part of a set phrase, such as 'not only... but (also)': _Paul can not only sing well, but also paint brilliantly_.
> https://en.wiktionary.org/wiki/cannot#Usage_notes