The active url for the .pdf for the species Atropoides_occiduus is mispelled as occidus, so that it cannot be downloaded with the correct name occiduus.
In the get_who_snakes.R script, I changed the name of the species in the snakes dataframe with the following line of code, just after loading it in the environment, in order to download the respective pdf image.
snakes[ which (snakes$species == "Atropoides_occiduus"), c("species", "split_spp")] <- "Atropoides_occidus"