Welcome!

There are many reasons for this problem

1. The Facebook page in the 'Meta for Developer' did not get the permission ("pages_manage_posts" and "pages_read_engagement")

Solution:

a. Go to 'Meta for Developer' and click 'My Apps'

b. Go into the app

c. Go to "App Review" --> "Permissions and Features". Search the permission and click "Request advanced access"

2. Because this login user is not registered developers from app roles. In Nov 2023, all unregistered developers need to register a developer account and add into app roles.

Solution:

a. Go to 'Meta for Developer', click 'My Apps' and Go into the app

b. Go to "App roles" --> "Roles" and click "Add People"

c. Choose "Administrator" and enter the Facebook username. 

d. If the Facebook user is this Facebook Page Owner, it will occur a error call "Unable to add a user with a role on the app's owning business. To add this user to the app, go to Business Manager and assign assets and access." at right bottom. To solve this problem, click "Edit roles in Business Manager"

e. Go to "Users" --> "People", find the owner name and click "..."

f. Choose "Edit business account permissions"

g. In Full control section, choose "Manage", so that this owner can use all the facebook API

h. Go to website, click 'Clear Facebook Access' and 'Login' again