There was a spelling mistake in the repos proposed by a previous answer (I've now fixed this).
Try:
install.packages('dplyr', repos = 'https://cloud.r-project.org')
The root cause for your problem is that your CRAN mirror is incorrectly specified.
You can try to fix this by adding the following line of code to the top of your script, or in your .Rprofile.
options(repos = c(CRAN = 'https://cloud.r-project.org'))
If you are using RStudio as your IDE, you can also fix this in the global options, under Tools > Global Options > Packages