It’s indeed not compatible with your OS, and I’ve run into the same issue on my mac mini (late 2012) version. I’ve explored a few different variants on how to deal with this, and some of this has been documented in this forum). The gist of it is as follows:
- You could either use an older version of Obsidian with matching main and installer version of Obisidian, or allow Obsidian to update still running on the older installer version (with possibilities for mismatch in promised and experienced functionality)
- (Get upgraded hardware…
Or install Linux on it… ) - Use Open Core Legacy Patcher to actually upgrade your OS to something newer, which allows Obsidian to be happy installed again
- Install Docker engine on your computer, and the linuxserver.io Obsidian package, and connect to it using a web browser. This allows you to connect, with some smaller keyboard issues, to an updated version even when your main OS can’t run it
I’m currently doing both of the latter options. I’ve upgraded my mac mini to now run on Monterey (12.6(?)), and I could upgrade further if I wanted to, using the Open Core Legacy Patcher. In parallell that computer is also running the docker engine, allowing me to connect to my main vault(s) from other devices in my apartment (without needing to sync 
).