没辙,无奈 sometimes have the same meanings. but the uses are different.
eg: the car has broken down.we 没辙(here is used as a verb),means we can't fix it or we can't do anything about it.
the car has broken down.we are 无奈(here is used as a adj.)implies much the same meaning ,we feel helpless,we can not find a way to solve this problem.
无奈can also used as a adv.
eg:我们无奈地走路回家。means we have to walk home reluctantly, because we can not do anything about the broken car.
HOPE THIS CAN HELP.