http://cran.rstudio.com/bin/linux/ubuntu/xenial/

E: Malformed entry 55 in list file /etc/apt/sources.list (Suite)
E: The list of sources could not be read.

http://cran.rstudio.com/bin/linux/ubuntu xenial/

The repository 'http://cran.rstudio.com/bin/linux/ubuntu xenial/ InRelease' is not signed.