When logging in to Walled Garden for the first time you will be prompted to "Accept Permissions" via the Facebook login prompt. All permissions must be accepted in order for Walled Garden to run ads on your behalf and using your Business Page.
If you modified the permissions granted, or did not accept all permissions, you will not be able to run ads on Walled Garden.
You can click here to be taken directly to the Business Integrations page or re-accept all Walled Garden permissions by following these steps:
FACEBOOK DASHBOARD:
Click on the profile photo in the upper right corner.
Click on Settings & Privacy
Then, click on Settings
On the left menu, click on Security & Login
Then, click Business Integrations
Find Walled Garden, and click on View and Edit
Toggle ALL permissions ON. Click SAVE.
Then, log out of Walled Garden AND Facebook, and log back in. If you are still experiencing issues, you can remove Walled Garden from your Business Integrations and provide fresh permissions again.