This thread says this:
I want to reply to a topic, but it won’t let me!
If you are a Member, you need to use the post approval procedure to have your reply moved to this category.
This is pretty vague and could do with some detail. I went to reply to a thread and all I saw was this: 
No reply button anywhere. However, if we were to make a post in another category and ask for it to be moved to Discussion, it would break this rule:
Please do not circumvent the post approval procedure by posting your topic in an entirely unrelated category.
So what are we to do?
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Details on how to use Post Approval are provided in the forum Rules. This post is directly linked to in the About topic you’re referring to and explains how to use Post Approval for that category. Is the wording of the About topic unclear about where to find information on using Post Approval?
The full quote from this About topic is:
If you are a Member, you need to use the post approval procedure to have your reply moved to this category.
See: Post Approval Procedure in the Rules
The relevant quote from the linked post in the Rules topic is:
If you want to reply (not a new topic) to a topic that you do not have posting access to, please start a group message to Post_Approval , and mention the topic you want to reply to, as well as the body of your reply underneath. The people in Post_Approval will then check your reply and move it over shortly.
Can you clarify where the confusion is?
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My bad, I apologise. I didn’t see it, but now I know- thanks. 
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Update on this: post approval restriction for Discussion replies is removed, so above information is outdated. As a Member you can freely reply to Discussion topics now.
https://devforum.roblox.com/t/discussion-category-grant-members-access-to-reply-and-or-create-topics/253905/11
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