Certain Mac users have had trouble with the Itch version of the game. Sorry about that.
For some it was just the executable permission bit was lost when the game was originally packaged, but we fixed that and reuploaded the game back on June 8th.
For others it's an issue with some of the later versions of MacOS Gatekeeper. There's a semi-complicated way of setting Gatekeeper to allow apps from anywhere to run, but that's arguably a security risk, and for a couple Mac users it didn't work.
The one thing that has always worked for any Mac user with an problem has been downloading the game from a non-Itch source (i.e. Steam or GOG). Please email riotsupport@means.media, preferably with the email account tied to your Itch account, and we'll send you a key for one of those right away if that interests you.
Sorry again it didn't work right off the bat.