In order to add Facebook Line Items to Campaigns of an Ad360 Advertiser, you first need to link this Advertiser with a Facebook Account
The list of available Facebook Accounts you can link will be displayed in the drop-down menu named "Facebook Ad Account".
To be included in the list, the Facebook Account must have been linked in the Ad360 Account settings first. Plus the Facebook Ad Account needs to be active.
For more information on how to link a Facebook Ad Account to your Ad360 Account, read this article: Facebook Accounts Integrations