Greetings!
I am not sure if you still got the problem, as your posts seem to work now.
If you still got trouble, feel free to contact me via PM.
Keep in mind that you need to enable the osu! MSN integration if you are playing in Fullscreen mode.
But as I can see on the Screenshot this is not the case, so this shouldn't be the problem.
Looks like osu!Post couldn't recognise the running osu! instance, in rare cases if osu!Post is at fault restarting osu!Post helps.
I am not sure if you still got the problem, as your posts seem to work now.
If you still got trouble, feel free to contact me via PM.
Keep in mind that you need to enable the osu! MSN integration if you are playing in Fullscreen mode.
But as I can see on the Screenshot this is not the case, so this shouldn't be the problem.
Looks like osu!Post couldn't recognise the running osu! instance, in rare cases if osu!Post is at fault restarting osu!Post helps.