Not all social networks let you post content outside of their website or app.
For example, Facebook lets you post to your Facebook Page or Group but not your own personal timeline.
To see if the account you added lets you post content from Hopscotch:
If you are on a mobile device or tablet, open the right sidebar by clicking on the icon on the right side of the navbar at the top of the screen.
On the right sidebar, click the title bar of the social network of the account you want to switch to (i.e. Twitter).
Click on the social media account you want to switch to under that social network.
If you are on a mobile device, open the left sidebar by clicking on the icon on the far-left side of the navbar at the top of the screen.
Click on Timeline in the left sidebar.
If you see a New Post button at the top of your timeline, you can click it to post with that account.