The developer cannot be verified is usually a settings issue. By default macs now only like to run apps they know of. Go here and type.in the terminal command listed to disable this feature. I've been supporting Macs in a professional environment for 17 years. This will. Ot hurt your machine. There used to be radial button options to allow from Any, but Apple got rid of that as a default option. This will allow you to install any app without question.
https://www.google.com/amp/s/www.imore.com/how-open-apps-anywhere-macos-catalina...
Something else I know about itchio apps on Mac is that they tend to prefer being run from the itchio app. Which is kinda like Steam client but for itchio. Get it and run the app through it on it mac and you should be all good.