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.
I think I have already told you what is wrong. One of the LaTeX command you use is not found during LaTeX compilation. It does not exists among the loaded package. This is this line:
by adding the computed $b_{i}$ to $\mr$.
This syntax means :
I am writing LaTeX Math
I am using the \mr LaTeX command.
What is \mr ? From which CTAN package it comes from ? Are you trying to use a LaTeX command here ? Or do you want to do something else ?
Anyway, I said to you before the issue is here, and you did not even answer my last question.
Please next time, take the time to correcty explain what you want to do so that you can get some useful help