Dear Eren Beytas,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and I would like to convey that I have tested the behavior at my end by creating a community in the Viva Engage, where I am able to restrict the users in the Community from the posting the posts:


Since you are the community admin in the community but not able to find the settings, I would like to confirm whether the same problem happens when you access the community in the InPrivate or Incognito window of the browser.
If the same problem happens, I am sorry to convey that the problem should be further checked by the related development team where they can collect the HAR logs and find out the cause of the problem.
So, I request you to connect with your IT admin of your organization (How do I find my Microsoft 365 admin? - Microsoft Support) and raise a new service request from the Microsoft 365 admin center so that the related development team will connect with you and investigate the issue further.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: We apologize for redirecting you to related development support team because the moderators in this community are limited with the resource for further investigation and to get a quick resolution to your problem, we requested it.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia