@fialamiri Same here on a Mac.
In my case, I was able to reach the initial Aria settings menu once to select local AI models to download, but it seems like when I switched to another application at this point and switched back (I may have also restarted Opera at this point), Aria settings seems to not let me browse local AI models to download anymore nor enable the toggle to enable local AI model usage like you're saying.
Maybe there's an unexpected conflict in which Aria settings requires a local AI model to be downloaded and available before we can enable the toggle for local AI model usage, but we are no longer given the option to download local AI models so consequently we're stuck in this state.