Hi there,
Unfortunately nvhdab is correct- you'll need to stay on 10.14 or below (Flash Builder’s system requirements specify 10.6, 10.7 and 10.8 as the only officially supported versions). If you have to upgrade to Catalina, then we recommend investigating a third-party IDE, such as IntelliJ Ultimate which has both AIR and Flex support.
Hope this helps clear things up, Madison
... View more