For future reference this would be a proper reproducible example for your issue, you are supposed to ask questions providing sample data, library calls and relevant code to reproduce your issue. (all this is explained in the link I gave you before, you just have to read it more carefully)
# Library calls for the packages you are using
library(dplyr)
# Sample data on a copy/paste friendly format
GBR <- data.frame(stringsAsFactors = FALSE,
X.Year = c(1991, 1991, 1992, 1992),
Rainfall..mm. = c(123, 78, 200, 129))
# Relevant code that reproduces your issue
GBR %>%
group_by(X.Year) %>%
summarise(Rainfall = sum(Rainfall..mm.))
#> # A tibble: 2 x 2
#> X.Year Rainfall
#> <dbl> <dbl>
#> 1 1991 201
#> 2 1992 329
Created on 2019-07-24 by the reprex package (v0.3.0.9000)