I can not send PM for some users, when I click on most users appears " start a conversation " but in some cases this option does not appear. How can I send PM in these cases?
If that poster has set his/her Conversations settings to "Accept conversations from people you follow only" -- and you are not someone they follow -- then your only option is to post on their profile page. But keep in mind, a post on the profile page is public; anyone can read it. You might ask them to PM you, as then you will be able to reply.