There are several variables to consider and since you're new to Moodle, perhaps the terminology is being confused. Let's try this:
- Enter your course and turn editing on.
- Choose to add a new Forum (forget about the one you already created for now)
- Provide a name and introduction for the forum, and leave the Forum type set to Standard forum for general use (the default)
- Leave all other settings/options at their defaults
- Click Save and display at the bottom of the page. You'll see something similar to the image below (hopefully). At this point, since there are not posts yet, there is nothing to reply to.
- Click Add a new discussion topic
- Provide a name and message, then click Post to forum. In the image below, the initial post now appears for users to read (they have to click to enter the thread and read the post).
- From the student's perspective, they will be able to enter the post and see the reply option (see below):
So, if you've been following those steps and students still cannot reply, I'm not sure what to suggest as a solution.
But, you mentioned you allowed "a student user" to try to reply to your post. Did you enroll the student with a student role, or as a guest? I don't believe that users with the "guest" role can participate in forums (they can't reply or post).
Remember, too, that the News forum is unique, in that it does not allow for students to reply (it's a one-way push of information, so "forum" is a bit of a misnomer).
Other factors that may prohibit posts and/or replies include permissions, blocking, groups, and access restriction settings.
I hope this is helpful to you, but feel free to ask more questions and provide us with more details so that we may better understand your predicament. Screen shots (captures) can be very helpful, too.