Hi,
ok, I see, actually I found a forum post with the same issue as yours: Mac 10.15 Catalina: "cannot be opened because the developer cannot be verified"
What I did just now on my macbook (and it works) is running the following in the terminal:
curl -O https://root.cern/download/nightly/root_v6.26.99.macos-14.0-x86_64-clang120.pkg
Then you follow the installation steps in a new window.
By default, your new ROOT executable should be installed in the Applications folder. Then from there you will still need to source thisroot.sh file as:
source /Applications/root_v6.26.99/bin/thisroot.sh
Then you can just type root in your terminal in order to start an interactive session.
Cheers,
Marta