Yes you can, although you must have reasoned and duly documented reasons. For first-semester subjects, you need to make your request before 31st December, and for second-semester, before 30th April (art. 9.1 in the Academic Regulations).

The reasons for total or partial registration cancellation may be: serious illness, sudden change in work or family situation, obtaining a place at another university, visa refusal or any other similar reason.