I am getting
Quitting from lines 42-58 (movielens.Rmd)
Error in download.file("http://files.grouplens.org/datasets/movielens/ml-10m.zip", :
download from 'http://files.grouplens.org/datasets/movielens/ml-10m.zip' failed
so I cannot duplicate your problem. Some sample data might help. A simple dput() might do.
It looks like you have a LaTeX problem rather than a strictly R /Rmarkdown problem but I do not know enough about how tinytex and R work to make a guess about where the problem is. At a wild guess, "geometry:margin=1in" looks like your normal weird LaTeX command that requires a latex package not necessarily included in tinytex.
Personally I have no use for tinytex and simply run a full installation of texlive . Trying that might help but it is a real pain to change LaTeX versions.
Sorry to be of so little help.