I used to have Twitter activated as a sales channel but today I just noticed that it's no longer an option. Even on the "Add sales channels" popup, it's not listed as a channel I could activate. That's weird! Did Shopify remove this feature and I somehow missed the announcement? Was it related to upgrading to SSL? (I had to remove the Pinterest channel to make my SSL work, was Twitter somehow caught up in that?)
Twitter has been phasing out the buy button for a while so the twitter channel on Shopify has also be deactivated as a result. So you've done nothing wrong - just that Twitter has pulled the plug on it.
There's an article here that covers some info on the twitter side of things.
The good news is there's other social channels that you can use with far more traction and engagement.