Error in seas(ts(y)) : start year of 'x' must be > 999.

season0<-ts(seasonal[,-1],frequency=4,start=c(1981,3))# no need to the `end` argument here, `start` and `frequency` are enough. N. 3 in the `start` argument refers to the third quarter of the year (date starts at 01-07-1981).
season0.seas<-seas(season0) # this will be run without error (have a look at season0 data to see the structure). 
plot(season0.seas) # and the plot.