The specific error I'm getting is that it says, "this program can't open" or something like that, and you can only press okay; it doesn't let you see what's wrong with it. I also think it isn't a security issue because those can easily be solved in privacy and security settings.
Another user suggested this fix:
"I usually have to drag the downloaded app into "Applications," then control+click the icon. It'll say it doesn't trust the origin of the file, but there should be an 'open' option anyways. It might take a couple tries to get it to show, but it's always worked for me so long as it's in "Applications" and not "Downloads.""
If that doesn't work, try installing the itchio app and download the demo through that.
I found a fix as I was also having the same issue!
1. Move the application to your Applications folder.
2. Open Terminal.
3. Paste
sudo chmod +x /Applications/RadiantsDemov2.1_Mac.app/Contents/MacOS/The\ Radiants\ Alpha\ Build
then press Return/Enter.
4. Enter your password when prompted and press Return again.
5. Open the Radiants application.