I just created a new Rmd file in RStudio with File | New File | R Markdown ... and indicated that I wanted to create an html file. I can knit that with the knit button.
You should also be able to do so. If that is the case then you made an error in the Rmd file and most probably in the YAML header: the first part of the file between the two ---lines. Can you show your YAML including the --- lines?
Regarding the PS part:
I experience the same when I handled the new Rmd file in this way: no html output. Looking at the documentation I saw that an output argument is expected. When I entered
markdownToHTML(‘filename.md’,output =‘filename.html’ )
the html file indeed was created.
I expect that with this correction your PS statements should have the same result as the knit button: so after correcting the Rmd file (still assuming there is something wrong there) both method should work.