If you encounter problems creating placements, then you should check whether the Audience Network is already set up in an app’s dashboard (it will be listed as a product on the left side and there will be no “Set Up” button in the corresponding space).
In this case you should add your app to property in Business Manager first and only after that you can create all the required placements. Otherwise, placements’ codes will not have correct App ID prefix and thus prevent reports from proper importing.
First, remove your app from the other property (if it is assigned already). Click on the Trash icon next to your app name inside that property.
Next, go to Business Manager → Business Settings → Data Sources → Properties https://business.facebook.com/settings/monetization-properties/ and add a property for the app (blue button “+ Add” on the middle pane).
And finally, hit “Add Assets” button on the right side and choose an app you want to use in this property.
Please refer back to the main instructions and follow the next steps. Recreate placements in the case of codes mismatch.