Flipper cannot be opened because the developer cannot be verified in macOS Catalina
Just Right Click > Open. Or alternatively, open System Preferences, Security & Privacy, General tab, and Click 'Open Anyway'. Guide on support.apple.com
This is a general problem (security feature called Gatekeeper) with apps on macOS where the developer is not registered with Apple (Paying $99/year). Specifically, Gatekeeper saves a quarantine flag on a downloaded file. So alternatively, you can remove this flag for your specific file, or disable Gatekeeper entirely so you never have this issue again.
A nice explanation by the Terraform team who faced the same issue:
Indeed the new version of MacOS is stricter about running software you've downloaded that isn't signed with an Apple developer key. Terraform distributions are signed with a HashiCorp key with signatures distributed out of band on releases.hashicorp.com, but MacOS Catalina is requiring participation in Apple's developer program specifically, and Terraform is currently not built to support that. source
Go to System preference, Security and Privacy and choose Opneanyway:
You can run following command to remove the warning and allow un-authorized app to run on macOS Catalina:
xattr -d com.apple.quarantine /Applications/Flipper.app