Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers.
In this case, however, the first example from the seasonal::seas help page clearly shows the problem
library(seasonal)
library(ggplot2)
m <- seas(AirPassengers)
autoplot(m)
#> Error: Objects of type seas not supported by autoplot.
Created on 2020-04-06 by the reprex package (v0.3.0)