I searched for a while but only found this:
Oh, and by the way: The "unknown compression method" result may be a bit misleading. This merely signals that zlib was unable to determine how to decompress the given data and this is more often than not caused by 'broken' input that, at least in the given form, cannot be processed by zlib in the first place.
So, it's probably one of three things:
1. Download error, redownloading will fix it.
2. Disk error, hard drive (or SSD) is getting close to failure (more likely if it is older - in any case, if you haven't made any backups recently, I recommend backing up your most important data just in case it is this reason - ideally to an external drive or to "the cloud").
3. Python, for some reason, is unable to use the specific zlib codec. Looking this up seems to happen with various different Ren'Py games, like this one or DDLC. You might be able to install zlib manually? This is unlikely to fix it, but on a Mac you can try run:
xcode-select --install
And it should install all the needed files. But I doubt it and am unable to test this. Ideally, try redownloading first (and in case drive is failing, also make backups of your other important data).