Don't worry. I had the same issue, and it's very easy to bypass.
To start, open Terminal. Once it has opened, copy-paste this into Terminal:
xattr -d com.apple.quarantine
After that, add a space, and drag Twine into Terminal. You should now see that the path to Twine was automatically added.
Press enter, and it should automatically open. If not, just open up Twine regularly, and it should open up.