ggsave saves only the most recent plot. So either you assign it to a variable first:
e.g.: plot = ggplot2:: autoplot(holt_method) + ## Print in order to display in plots window
ggtitle(paste(key,pair))
...
and then followed by ggsave(plot = plot, paste0("~/Desktop/",key, pair, ".png",
width = XX, height = YY, units = "cm") (or "in" if you prefer) (I usually define the size, as the automatic save sometimes leads to strange results)
or maybe you can wrap everything you have now in the print() into the ggsave() function. Not sure, never tested this.