MacOS Catalina protects you from malicious code by asking for all Installers to be Notarized. This means sending the files to Apple, where they scan for defects.
Unfortunately this scanning requires all contents be 64-bit, clean. This would mean dropping support for perfectly good setups like:
Big Business 12 Old Client on El Capitan or Sierra
UPS, FedEx, etc. using 32-bit ODBC
Microsoft Outlook, etc. installed 32-bit
For now, please circumvent GateKeeper on macOS Catalina as follows. (6986)