wa alykum asalam warahmatulahi wabarakatuh.
i think its because your a limited member, this is because you got less than 50 posts. so just try to post in the topics you can and insha Allah soon you'll be a full member and be able to post wherever apart from the bros section insha Allah. if you still dont understand plz reply bak and ask wa ur unsure of.. jazak Allah khayr.
wasalam o 'alykum warahmatulahi wabarakatuh.