For some custom audiences, reach estimates are no longer visible. This is an ongoing issue on Facebook side.
Facebook has temporarily removed the ability to see reach estimates via the API for the following Custom Audience types:
1. Custom Audiences from a customer data file
2. Offline conversions Custom Audiences
3. Website Custom Audiences that use Advanced Matching
4. Mobile App Custom Audiences that use Advanced Matching
You will still be able to create new Custom Audiences, and use them for ads targeting. Ad delivery and reporting will remain unaffected by this change. We are currently investigating solutions to restore these reach estimates.
More information about this can be found on Facebook directly!