Facebook has recently changed its API and is no longer allowing pages with fewer than 2,000 likes to install custom tabs.
If your page has less than 2,000 likes you might not be able to install your promotion to your Facebook page. You will see the following error when attempting to install.
If you are still receiving an error you can also try this link: https://www.facebook.com/dialog/pagetab?app_id=142973449096741&next=https%3A%2F%2Fwww.aptivada.com%2Fdashboard.
It will bring up a landing page where you can select the Facebook Page you want to install the Promotions tab to.