Knit RMarkdown to PDF not working

Hi yihui,

i have an issue with knit rmarkdown to pdf in RStudio. I am using TinyTex and try to knit following rmarkdown example:

---
title: "Untitled"
output: pdf_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:

```{r cars}
summary(cars)
```

## Including Plots

You can also embed plots, for example:

```{r pressure, echo=FALSE}
plot(pressure)
```

Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.

Selection of "Knit to PDF" provides :

  |.........                                                        |  14%
  ordinary text without R code

  |...................                                              |  29%
label: setup (with options) 
List of 1
 $ include: logi FALSE



processing file: Test.Rmd
  |............................                                     |  43%
  ordinary text without R code

  |.....................................                            |  57%
label: cars
  |..............................................                   |  71%
  ordinary text without R code

  |........................................................         |  86%
label: pressure (with options) 
List of 1
 $ echo: logi FALSE

  |.................................................................| 100%
  ordinary text without R code


"C:/Program Files/RStudio/bin/pandoc/pandoc" +RTS -K512m -RTS Test.utf8.md --to latex --from markdown+autolink_bare_uris+ascii_identifiers+tex_math_single_backslash --output Test.tex --template "C:\Users\v005194\Documents\R\win-library\3.5\rmarkdown\rmd\latex\default-1.17.0.2.tex" --highlight-style tango --latex-engine pdflatex --variable graphics=yes --variable "geometry:margin=1in" --variable "compact-title:yes" 
output file: Test.knit.md

tlmgr search --file --global "/xcolor.sty"
Trying to automatically install missing LaTeX packages...
tlmgr install xcolor
No connection to the internet.
Unable to download the checksum of the remote TeX Live database,
but found a local copy so using that.

You may want to try specifying an explicit or different CTAN mirror;
see the information and examples for the -repository option at
http://tug.org/texlive/doc/install-tl.html
(or in the output of install-tl --help).

tlmgr.pl: package repository http://ctan.math.washington.edu/tex-archive/systems/texlive/tlnet (not verified: unknown)
[1/1, ??:??/??:??] install: xcolor [17k]
TLPDB::_install_package: downloading did not succeed
tlmgr.pl: package log updated: C:/Users/v005194/AppData/Roaming/TinyTeX/texmf-var/web2c/tlmgr.log
tlmgr search --file --global "/xcolor.sty"
! LaTeX Error: File `xcolor.sty' not found.

! Emergency stop.
<read *> 

Fehler: Failed to compile Test.tex. See Test.log for more info.
Ausführung angehalten

I am not using the xcolor-package anywhere... What can i do to fix that problem? I am an simple user on my work-computer (not an admin).
I see a possible solution at ur website https://yihui.name/en/2018/08/tinytex-flash-drive/. But I am not able to use Dropbox on my computer.

By the way: I started this thread at https://tex.stackexchange.com/questions/459113/problem-with-knit-r-markdown-to-pdf too.

Review the reported TinyTeX and RStudio knitting command, including the failed xcolor.sty lookup and the Test.log reference. Determine whether the report identifies a reproducible TinyTeX problem or only an environment-specific package-download failure; a useful outcome would require a confirmed fix or a clearly documented limitation.