Ideas to try out:
a) permission denied
Try to read all PM and linked threads there. May be you lack some permissions cause did not learn first steps yet.
b) to change forum GUI theme
Preferences → Interface → Theme
The current forum edition has 4 themes.
3 of them has “start new thread” button with floating position over other elements - on a different layer of HTML markup. In rare cases that could be a problem of button disposition: the button could be located outside of visible and scrollable viewport of browser window (bug of browser engine or frontend website developers):
1 of them has solid in-main (alongside with payload page content) layer position (currently the Dark theme), which “add new thread” button disposition a way less possible.
c) turn off all plugins
May be some forum plugins behaves badly. Try to temporarily to turn all them off (https://meta.discourse.org/t/how-to-use-discourse-safe-mode/53504) by visiting the Manjaro Linux Forum link.
d) call JavaScript for help: let it to find 'start new thread' button and to click on it for you
Substitute the URL address field value with the
javascript:(()=>document.getElementById("create-topic").click())()
and hit Enter.
New topic window expected to be appeared.
At least works on the forum engine version (just as technical note)
document.head.querySelector('meta[name=generator]').content
// 'Discourse 2.8.0.beta7 - https://github.com/discourse/discourse version cbd7898d1d70198f9fda0eac27a68affde5c8bb4'
After you tried, please welcome into Did you experience problems to start a new thread (topic) on the forum?