Facebook recently changed its API and is no longer allowing Pages with fewer than 2k Likes to install custom tabs.
If your Page currently has less than 2,000 followers, you might not be able to add POWr to your Facebook Page yet. You will see the following error during the install steps:
Once your Page has enough Likes, you should be able to add any POWr plugin to your Facebook Page.
Update (2019-05-18): Facebook bug resolved
Facebook announced on their Developer Support forums that the issue with custom tabs not installing on pages that do have more than 2k likes / followers / fans is now resolved.
Update (2019-04-12): Facebook bug
Facebook is currently experiencing an issue where installing a custom tab is not working for users and pages with more than 2k likes. You can track the issue here: https://developers.facebook.com/support/bugs/1978223865639904/