Accessing Facebook Ads custom conversions data requires an extra Facebook Ads permission.
Due to this, you'll simply need to reconnect your Facebook Ads integration before you can pull in conversions data. Here's how to do so.
How to access Facebook Ads custom conversions data
First, click on Facebook Ads on your Integrations dashboard.
Then click "Switch Account".
Click "Connect New Account" at the bottom of the screen.
Then follow the prompts to log in via Facebook. When doing so, ensure that you accept all permissions.