Right-click + Open resulted in the same error dialog; I presume this has something to do with Big Sur. The option to allow apps from "Anywhere" is also not available in the preferences pane.

Right-click + Open has to be done twice to work.

The anywhere option is hidden by default, use sudo spctl —-master-disable from Terminal instead, which flips Gatekeeper settings to Anywhere.

Edit: there's also this path: https://apple.stackexchange.com/questions/415713/verify-code... which doesn't seem to activate Gatekeeper as part of the install process.