[Jeff Sayre]

The integrated version of bbPress creates group-based forums. So, to add a topic, you must be within that group’s forum. If you want a sitewide forum, you will have to install another copy of bbPress (you can use the same bbPress tables) using deep integration. There are a number of threads on these forums about how to do just that.

Read this Codex article for more information:

https://codex.buddypress.org/developer-discussions/buddypress-forum/