Check the selected channel
Studio sends social posts through Buffer. It lists channels from your Buffer account. To use LinkedIn through Buffer, set it up there. The native LinkedIn setting is for a separate connection.
Check the preview and channel options before you send. Check that each channel accepts your media. Write for the people who will read the post. Test links and review images too.
Each chosen channel gets the same post text. Make separate posts if you need different wording. A preview helps you review a post. The service can still reject a file or option.