Opentoonz isn't officially signed and that will trigger most antivirus software.
This is true on Windows as well as Macs and you should have an option to accept the risk and continue.
To date, 10 years and going... there have been exactly zero confirmed reports of malware included in Opentoonz.
You do want to only download from official sources of course and there is one site that should be avoided.
It will be obvious if you visit that site and of course I won't link that here.
That is also where you can find the nightly releases.