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Go to the apple icon in the top. -> System Preferences -> Security & Privacy -> General.
There should either be a notification if you just tried to open something and it was blocked like you described. Here you can open it anyway, followed by a popup to realy realy open the file :)
Same for me. got it working on windows but osx gets stuck in "Creating Environment". This message updates to a stuck "Installing Vanilla Launcher" after 1 navigation click.
Closing the installer and reopening. Most of the time I can restart the download. Sometimes it 'thinks' the modpack is...