Cannot, in the sense of "am unable to" or "am not permitted to" is written as one word.  Whenever we use the contraction can't, we are contracting the one word cannot.

Thus, for example, if I want to express the idea that I am unable to work here or I am not permitted to work here, I must write, "I cannot work here."

The only instance in which "I can not [two words] work here" would be correct is this:  I have a choice between working here and not working here.  In other words, I can work here or I can not work here.  In this case, not negates work; in cannot, not negates can.  The latter is what we usually mean when we say we cannot (can't) do something.

Strange as it may sound, "I can not work here" means "I am able to not work here," whereas "I cannot work here" means "I am unable / not able / not permitted to work here."