Automatically activate a new user after they verify their account by email
I cannot seem to find the option to allow a new user to start posting after they receive the verification email?
Before I manually activate them in the wp user menu the user receives a message ERROR: User has not been activated.
Wordpress handles all that, not sure what you exactly mean.
The user registers, gets an email for verification, he clicks, selects a password and that it is.
wpForo doesn't do any of this. As soon as the user is logged in, he can post.
Maybe some moderation will be needed for the first posts, still this is configurable in wpForo.
Probably you have other plugin which add a new step "Activation" step in user registration. I'd recommend disable it to test, or contact that plugin support team. This could be a profile builder plugin like BuddyPress or user approval plugin.