nope you can enrol, but i wouldn't do it online for ROTP. i would go to your nearest recruit office
i wanted to be a nurse and was told i could enrol in ROTP as soon as i graduate HS
but changed my mind and was offered an Armoured Soldier MOC.
however theres a difference but its not something youll notice
when you enrol as soon as you graduate you wont get any bonuses really however they WILL pay for your school
when you enrol in your last year you get a certain about of money that will go towards your tuition debt [just assuming you wont have it paid off]
when you do enrol you are required to serve when you get out. so if you're having any doubts i would wait til either your last year or 2nd last year of university to keep thinking about it
good luck
hollyy