Only if you are an admin on a Facebook page are you able to connect it to POP
In order to make a connection between your Facebook page and POP, you need to make sure you are an admin of the Facebook page.
'Admin' does not mean you need to be a user on the Facebook page, you need to have the role 'Admin' on the Facebook page.
In order to check this, you can navigate to your User Roles on the Facebook page.
Navigate to your Facebook page, click Settings in the top right and go to User Roles in the left sidebar.
There you can see the full list of existing page roles and people who have access to your Facebook page.