This can happen for a couple of reasons. One is if you leave the tab open for a long time without responding. You usually just need to copy your response, do a refresh, and paste the answer back in and submit again.
Another reason is if you have replied to too many posts in a short period. Atlassian thinks you might be a bot and so slows you down. You have have to wait about 15 minutes or so, refresh and then post your answer again (like above, copy your answer before hitting the refresh).
Finally, if you have indeed posted this same answer to another post, again, Atlassian thinks you might be a bot and doesn't allow that. Be sure to modify the answer in some way so it is not exactly the same. It also might have already flagged this one as being sus so you might have to change the text up AND wait about 15 minutes.
If none of those are happening and you still get it. Try another browser or Incognito mode. If still an issue, post back here.