I can only guess it wants you to pass them as parameters before the list rather than as magic numbers e.g
buff_dist1 = 100
buff_dist2 = 200
buff_dist3 = 300
distanceList = [ buff_dist1' ....
Also be aware your "riverbuffer" + ".shp" needs to change name during the iteration to reflect the input distance. Currently it will just overwrite with the same name.
Use something like
"Riverbuffer_" + str(distance) + "_ft.shp"