1) 'for' of enrol for indicates the object, aim, or purpose of an action or activity
EX: enrol for school/enrol for the course/enrol for special offers
2) 'in' of enrol in indicates placing one's name in a roll, list, or record
EX: enrol in school/enrol in the course
If the object of 'enrol', say, school or course, refers to a list you can add your name to, then use either 'in' or 'for'. If the object of 'enrol' does not have a list you can add your name to, then you can't use 'in'
EX: enrol for special offers (OK)
EX: enrol in special offers (Not OK)
'special offers' doesn't have a list you can add your name to. That is, you register to get (i.e. for) special offers.