You are missing a + between your geom_bar and your geom_crossbar.
library(ggplot2)
data_sum <- data.frame(group = c("control", "food.inhibition"),
weightchange = c(-0.08, -0.4))
ggplot(data_sum, aes(x=group, y=weightchange)) +
geom_bar(stat = "identity", fill="skyblue") +
geom_crossbar( aes(x=group, y=weightchange, ymin=weightchange-1.52,
ymax=weightchange+1.52), width=0.4, colour="orange",
alpha=0.9, size=1.3)

Created on 2020-04-02 by the reprex package (v0.3.0)