Loop in a .csv file with IF cases
I would consider wrapping the conditions up inside a function:

def is_header(pattern):
    if pattern in header:
        pattern_index = header.index(pattern)
        if fields[pattern_index]:
            pattern_key = "path" if pattern == "url_slug" else "keyword"
            payload['seo_page[{}]'.format(pattern_key)] = fields[pattern_index]            
        else:
            print("{} not defined for {}".format(pattern, fields[pattern_index])


Because the tests are essentially the same, you could create a common function and pass in the pattern to test against (url_slug or keyword). There is some extra hocus pocus to get pattern_key to be the right thing in order to point to the right key in your seo_page dict (I assume it is a dict?).