If you are talking about rich embeds, they currently don’t work with Twitter integration in Discourse:
As an alternative, this may be relevant to you:
If you are talking about standard oneboxes, I don’t know why it wouldn’t work natively unless you have changed onebox settings in your install, or your IP is blocked by Twitter for some reason.
I had this checked, and just unchecked it - but no changes in one boxing with twitter.
See here , below is what happens right now with twitter posts. I used to get the standard inbox, but perhaps I messed a config setting up? What might I have messed up?