Sounds like it wasnt constructed perfectly. So your last roof panel was left with too big a gap, although you dont give much info on the problem. This can be caused by not allowing for an uneven house wall in construction or unlevel sills at begining. You can buy a matching roof sheet and cut to fit. Taking the old one out should be straight-forward.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix roofing.