You don't say how you are running Sweave, but that looks like RStudio. To debug something like this, just run Sweave explicitly in the R console, e.g. if your input file is source.Rnw, run
Sweave('source.Rnw')
This will produce source.tex. Open that file in a text editor and look at the start of it. You will see that \Schunk is used on line 27, but \begin{document} doesn't occur until sometime later.
My guess is that you added some text or a code chunk to the header. All text belongs after \begin{document}.
Edited to add: It turns out from the comments below that you were using print(...) in a code chunk before \begin{document}. In Sweave, print output goes into the document. If you want a message to show up in the console log but not the document, use message("some text"). You'll also need to suppress the echoing of the command if you want to do this in the document header. For example,
<<echo=FALSE,results=hide>>=
message("Started at ", Sys.time())
@
will result in something like this in your console log:
1 : keep.source term hide (Untitled.Rnw:6)
Started at 2017-09-27 08:28:33
and nothing in the document.