从您的输出来看，pg.form 实际上是一个列表表单而不是单个表单.要访问第一个表单，请执行

From your output, pg.form is actually a list forms rather than a single form. To access the first form either do

set_values(pg.form[[1]], Month="8")

或者你可以做

pg.form <- html_form(html(pg.session))[[1]]

相反.