Update: You can use this IFTTT recipe to automate the process!
Twitter has rolled out some policy changes which have blocked a couple of third party services to sync with their platform.
LinkedIn is one of the affected services. They are no longer allowed to sync with Twitter platform.
If you have been syncing your Twitter content with your LinkedIn account, you would now have to use LinkedIn to post to the two networks simultaneously.
LinkedIn spoke about the new changes:
As Twitter shared earlier today in a blog post from Michael Sippey, they are increasingly focused on “providing the core Twitter consumption experience through a consistent set of products and tools.” Consistent with Twitter’s evolving platform efforts, Tweets will no longer be displayed on LinkedIn starting later today. We know many of you value Twitter as an additional way to broadcast professional content beyond your LinkedIn connections. Moving forward, you will still be able to share your updates with your Twitter audience by posting them on LinkedIn.