This problem can happen when you are signed in and “posting as” your page, rather than from your personal profile. Since you can’t add tabs to your page when posting as a page, you just need to switch to your personal profile.
To do to switch just click on the arrow that you see at the top right corner in Facebook, and choose to “Use Facebook as” your personal profile.
Next, login into your control panel and you will now be able to add Social Store to your page.
Feel free to comment here if you have any other questions.