Messaging is not automatically enabled on every Facebook page, and sometimes it might have been manually deactivated in the past. If you want to consolidate the messages for your Facebook page into Superchat, messaging must be enabled beforehand.
Here's how you can enable or disable messages for pages you have access to on Facebook:
Log in to Facebook and click on your profile picture at the top right.
Click on See All Profiles and select the page you want to switch to.
Click on the profile picture of your page at the top right on Facebook.
Click on Settings and Privacy, and then click on Settings.
In the left menu, click on Privacy.
Click Edit next to Messages.
Select either On or Off under Can others send messages to your page? Your choice will be saved automatically.
You can also find more information about this directly in the Facebook guide.