I'm running macOS Catalina 10.15 with Xcode 11.1 (11A1027)
The Mac Deployment Target option in Xcode is greyed out in one of my projects. In another project it's not greyed out and available.
Project A:
Project B:
Either this is an Xcode bug or I've incorrectly configured something.
Catalyst currently supports only making Mac versions of iPad apps. In Project A, you selected only the iPhone checkbox so the Mac checkbox is disabled. In Project B you selected the iPad checkbox, which enables the Mac checkbox.