i belive that you are only ever able to make a roller coaster 29 pylons long. but its
never been tried before so make sure that hight restriction is not a problem for geting
over buildings ect. even if near building somtimes wont let you curve around it so bring
the coaster out a little way or redirect.

Anyways I find it could be 3 things....

1) A part of the rollercoaster could be covering another ride or something in your park
that means you can't finish it until you replace or change the height of that certain part
before It will start working.
2) You have too many pylons on your rollercoaster.
3) You've ran out of money.
They're the only reasons I could think of because I haven't played it in ages.
Hope This will help :)