A forum need to be published to the learners before they can view it on the Learner portal.
There are two ways how to publish your forum:
If the forum is part of the course, just publish the course.
The forum will be available in the course content and on page Forum in the user portal.
If the forum is not attached to the course, it needs to be published using permissions tool.
The forum will be available on page Forum in the user portal.
Manage who can view your forum and post a comments to it:
You can also access Permissions settings from the Forum page.