You're right. They look identical, except for ylim(0,2.5), but ggplot is unconvinced. The message is a complaint that labs(y="Mean ISI ± SE", x="Site") isn't being tacked on to a ggplot object.
These are hard to eye-check and a little sample from anovadata would make a nice reproducible example, called a reprex.
Try
RichPlot
and see if it produces output, which is pretty good evidence that you do have a ggplot object, and v.v.
If that works
RichPlot + labs(y="Mean Total Species Richness ± SE", x="Site")
to see if you still get the error message.
If you don't, put spaces around <-