There are two possible explanations for why your student(s) can't post. First, do you have the Forum set to Visible Groups or Separate Groups? If so, the student(s) are likely not in a group. Also, did you set the appropriate grouping? If so, it might be that the groups are not associated with that grouping. For either, please see out 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.