In the (closed) thread "Cannot Knit to PDF" (Cannot Knit to PDF), user124578 reported the following error message
! Sorry, but C:\Users\naa56\AppData\Local\Programs\MIKTEX~1.9\miktex\bin\x64\pdflatex.exe did not succeed.
! The log file hopefully contains the information to get MiKTeX going again:
! C:\Users\naa56\AppData\Local\MiKTeX\2.9\miktex\log\pdflatex.log
Error: Failed to compile test.tex. See https://yihui.name/tinytex/r/#debugging for debugging tips. See test.log for more info.
Execution halted
I got the same error message using MiKTeX for the first time. I managed to solve this and am posting this for others like myself who don't know "TeX" or "MiKTeX" and are suddenly confronted with this error message.
The solution was to open the .tex file generated by RStudio in TeXWorks (default on double-clicking), then to click the green triangle on the button bar to compile to pdf. This resulted in a number of prompts, indicating that ".sty" files were missing with a button to install them.
That was the problem as it turned out. Once the missing "*.sty" files had been installed, I could "Knit Document" in RStudio in the usual manner. Hopefully this can help others in a similar pickle.