These calls are supported. I would have to have a closer look at Sample.docx to troubleshoot.
What do you get if you take it step by step and print the results?
like:
>>> document = Document('Sample.docx')
>>> document
<docx.api.Document at 0x121cebed0> # this looks good so far
>>> paragraphs = document.paragraphs
>>> paragraphs
???
>>> paragraph = paragraphs[2]
>>> paragraph
???
>>> paragraph.style = 'Title'
>>> paragraph.style
???
Also, getting the style name right is sometimes a trick. I don't see how that would cause the behavior you're seeing, but might be worth looking into. Are you sure you have the style 'Title' available in Sample.docx?