There are a few possible explanations for why your student(s) can't post. First, the student(s) might not be in a group. Go to the Participants page (Course Dashboard > Participants) and scroll to the student. It should list the group(s) they are in under the Groups column. If they are not in a group, click the pencil next to "No groups" and select the desired group from the dropdown. Then click the Save icon on the right. If they are in a group, did you set the appropriate grouping in the forum settings? If not, do so. If you do have it set to the appropriate grouping, it might be that the groups are not associated with that grouping. For either, please see our help guide on how to Create Groups and Groupings.
The other possible explanation is that you set your forum type to the Q&A Forum Type, but did not post any questions. When using a Q&A Forum Type you, the instructor, must post first. You are posting the "question." Students then come in and reply to your question with their "answer" (hence the name Q&A). Students cannot post questions, so if there are no questions posted, students are unable to post in the forum. Question posts are not brought over when importing or restoring course content, so they will need to be re-posted each term. You must post the question to each group for the students to be able to answer.