A variable/column name cannot contain spaces, which is why you get an error as ggplot don't know what to do with a series of words. As your data is loaded in as a data.frame with a column name featuring spaces, you could try putting the variable name in accent signs `:
ggplot(data = meanundernourisment) +
geom_point(mapping = aes(x = `Year`, y = `Prevalence of undernourishment (% of population)`)