I am on Catalina and have XCode 12 beta 6 and Safari Technology Preview installed but am getting the following :-
% sudo xcode-select --switch /Applications/Xcode-beta.app
Password:
% xcrun safari-web-extension-converter /Users/aarongray/Desktop/DevelopingASafariWebExtension
safari-web-extension-converter requires at least Safari 14.0 to run.
Unable to parse manifest.json at file:///Users/aarongray/Desktop/DevelopingASafariWebExtension
I have set the Safari Technology Preview as the defualt browser but that did not help.
Created as new topic :- https://developer.apple.com/forums/thread/660279
AaronNGray
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Have installed Safari 14 Beta for Catalina :-
https://developer.apple.com/download/more/?=Safari