The correct one is 'can't'. It is what we call a 'contraction'. It is made from combining the two words 'Can' + 'Not'. We put the apostrophe ( ' ) where we dropped the letter (Got rid of 'o' so put an apostrophe there). 'Cann't' is not actually a word. When we write can't we drop one of the 'n' letters, I'm not sure why but that's just how it is.

Me encantan los oceanos de bolivia

You can say either CAN'T (one word/one syllable) or CANNOT (one word/two syllables with the stress on the second syllable). 'Cannot' is more formal than 'can't'. 'Cann't' does not exist.

I can't (I can not)

i think it is "can't"