This is a common issue when the publishing option for the profile is unchecked.
To fix this follow the steps below:
In your account, click on your username in the upper-right hand corner, then scroll up and click on "Social Profiles"
Here you'll see all your connected social profiles
(don't see your account? click here to learn how to connect them)
Select which accounts you want to appear in Publishing by check marking the "Publishing" box
Once check marked, they will now appear in Publishing as options when creating/scheduling posts.