I have my privacy setting at “Friends and Followers” for PMs however, when I go on my alt that follows my account [ not friends with it ] I cannot send a Message.
Because it’s your alt that’s (trying to) send the PM. That’s just how ROBLOX works – both users’ PM settings affect whether or not they’re able to send each other PMs.
I still get the occasional PM I cannot reply to due to people’s settings. It gets even worse when they think I am ignoring them and they send messages asking if I received the previous message.