Just try this:

httrack <https://vjetropev.blogspot.com/> -v -s0 -F "Mozilla/5.0 (Windows NT
10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169
Safari/537.36"

p.s. and remember to place quotation marks - "URL" - around the URL (and
remove the angle brackets, which have been added by the forum software)