Check permissions from Control: Platform → Community → Search correct category → Edit category:

Check permissions from all the needed roles.

Hey ​@Zayn Jia perhaps this is a setting in your community category?

Check if the box “create topics” is checked for Registered Users or any other user roles it is happening for.

Registered users are allowed to create topics and replies. That’s not the issue!

This is just a hunch but have they confirmed their registration from email?

New community or old and issue just started?

Might be related to this?
 

I would assume that the new members to the community are not obtaining a role that was awarded prior. That role is likely tied to permissions on boards linked to the ability to post.

The primary roles permissions might have been changed on accident as well. That can be checked at the board level.

In admin, go to /users/overview and see what roles are being given to individuals.

Custom roles via API take about 2 hour to get assigned with Gainsight. There is not an automatic way to get these assigned after or during registration to my knowledge. If a custom role is needed in your community to post that could be the issue.