Facebook introduced new requirements which means that you’ll have to install the Planable app to your Facebook group.

Go to the Facebook group you're managing and find "General settings" from the menu on the left:

Then, on the main page, scroll down to "Manage advanced settings" section and click on the pencil icon next to "Apps":

Search for Planable, and click on the app to add it.

Once you're done with this step, go back to Planable and "Authorize" your group.